This adds support for the MV78230 based DS414 NAS by Synology. The
relevant bits have been extracted from the 'synogpl-5004-armadaxp'
package Synology kindly published, garnished with a fair amount of
trial-and-error.

Sadly, support is far from perfect. The major parts I have failed in
are SATA and XHCI support. Details about these and some other things
follow:

Device Tree
-----------

The device tree file armada-xp-synology-ds414.dts has been copied from
Linux and enhanced by recent U-Boot specific changes to
armada-xp-gp.dts.

SATA Support
------------

There is a Marvell 88SX7042 controller attached to PCIe which is
supported by Linux's sata_mv driver but sadly not U-Boot's sata_mv.
I'm not sure if extending the latter to support PCI devices is worth the
effort at all. Porting sata_mv from Linux exceeded my brain's
capacities. :(

XHCI Support
------------

There is an EtronTech EJ168A XHCI controller attached to PCIe which
drives the two rear USB3 ports. After a bit of playing around I managed
to get it recognized by xhci-pci, but never was able to access any
devices attached to it. Enabling it in ds414 board config shows that it
does not respond to commands for whatever reason. The (somewhat) bright
side to it is that it is not even supported in Synology's customized
U-Boot, but that also means nowhere to steal the relevant bits from.

EHCI Support
------------

This seems functional after issuing 'usb start'. At least it detects USB
storage devices, and IIRC reading from them was OK. OTOH Linux fails to
register the controller if 'usb start' wasn't given before in U-Boot.

According to Synology sources, this board seems to support USB device
(gadget?) mode. Though I didn't play around with it.

PCIe Support
------------

This is fine, but trying to gate the clocks of unused lanes will hang
PCI enum. In addition to that, pci_mvebu seems not to support DM_PCI.

DDR3 Training
-------------

Marvell/Synology uses eight PUPs instead of four. Does not look like
this is meant to be customized in mainline U-Boot at all. OTOH I have
no idea what a "PUP" actually is.

PEX Init
--------

Synology uses different values than mainline U-Boot with this patch:
pex_max_unit_get returns 2, pex_max_if_get returns 7 and
max_serdes_lines is set to 7. Not changing this seems to not have an
impact, although I'm not entirely sure it does not cause issues I am not
aware of.

Static Environment
------------------

This allows to boot stock Synology firmware at least. In order to be a
little more flexible when it comes to booting custom kernels, do not
only load zImage partition, but also rd.gz into memory. This way it is
possible to use about 7MB for kernel with piggyback initramfs.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Acked-by: Stefan Roese <sr@denx.de>
Reviewed-by: Tom Rini <trini@konsulko.com>