The original driver author seemed to be under the impression that a driver
cannot be removed if it does not have a .remove method. Or maybe if it is
a built-in platform driver.

This is not true. This crash can be created:

root@ubuntu:/sys/bus/platform/drivers/hisi-lpc# echo HISI0191\:00 > unbind
root@ubuntu:/sys/bus/platform/drivers/hisi-lpc# ipmitool raw 6 1
 Unable to handle kernel paging request at virtual address ffff000010035010
 Mem abort info:
   ESR = 0x96000047
   Exception class = DABT (current EL), IL = 32 bits
   SET = 0, FnV = 0
   EA = 0, S1PTW = 0
 Data abort info:
   ISV = 0, ISS = 0x00000047
   CM = 0, WnR = 1
 swapper pgtable: 4k pages, 48-bit VAs, pgdp=000000000118b000
 [ffff000010035010] pgd=0000041ffbfff003, pud=0000041ffbffe003, pmd=0000041ffbffd003, pte=0000000000000000
 Internal error: Oops: 96000047 [#1] PREEMPT SMP
 Modules linked in:
 CPU: 17 PID: 1473 Comm: ipmitool Not tainted 5.2.0-rc5-00003-gf68c53b414a3-dirty #198
 Hardware name: Huawei Taishan 2280 /D05, BIOS Hisilicon D05 IT21 Nemo 2.0 RC0 04/18/2018
 pstate: 20000085 (nzCv daIf -PAN -UAO)
 pc : hisi_lpc_target_in+0x7c/0x120
 lr : hisi_lpc_target_in+0x70/0x120
 sp : ffff00001efe3930
 x29: ffff00001efe3930 x28: ffff841f9f599200
 x27: 0000000000000002 x26: 0000000000000000
 x25: 0000000000000080 x24: 00000000000000e4
 x23: 0000000000000000 x22: 0000000000000064
 x21: ffff801fb667d280 x20: 0000000000000001
 x19: ffff00001efe39ac x18: 0000000000000000
 x17: 0000000000000000 x16: 0000000000000000
 x15: 0000000000000000 x14: 0000000000000000
 x13: 0000000000000000 x12: 0000000000000000
 x11: 0000000000000000 x10: 0000000000000000
 x9 : 0000000000000000 x8 : ffff841febe60340
 x7 : ffff801fb55c52e8 x6 : 0000000000000000
 x5 : 0000000000ffc0e3 x4 : 0000000000000001
 x3 : ffff801fb667d280 x2 : 0000000000000001
 x1 : ffff000010035010 x0 : ffff000010035000
 Call trace:
  hisi_lpc_target_in+0x7c/0x120
  hisi_lpc_comm_in+0x88/0x98
  logic_inb+0x5c/0xb8
  port_inb+0x18/0x20
  bt_event+0x38/0x808
  smi_event_handler+0x4c/0x5a0
  check_start_timer_thread.part.4+0x40/0x58
  sender+0x78/0x88
  smi_send.isra.6+0x94/0x108
  i_ipmi_request+0x2c4/0x8f8
  ipmi_request_settime+0x124/0x160
  handle_send_req+0x19c/0x208
  ipmi_ioctl+0x2c0/0x990
  do_vfs_ioctl+0xb8/0x8f8
  ksys_ioctl+0x80/0xb8
  __arm64_sys_ioctl+0x1c/0x28
  el0_svc_common.constprop.0+0x64/0x160
  el0_svc_handler+0x28/0x78
  el0_svc+0x8/0xc
 Code: 941d1511 aa0003f9 f94006a0 91004001 (b9000034)
 ---[ end trace aa842b86af7069e4 ]---

The problem here is that the host goes away but the associated logical PIO
region remains registered, as do the children devices.

Fix by adding a .remove method to tidy-up by removing the child devices
and unregistering the logical PIO region.

Cc: stable@vger.kernel.org
Fixes: adf38bb0

 ("HISI LPC: Support the LPC host on Hip06/Hip07 with DT bindings")
Signed-off-by: John Garry <john.garry@huawei.com>
Signed-off-by: Wei Xu <xuwei5@hisilicon.com>