Support qspi flashes for mx7dsabresd
1. introduce pin mux settings
2. enable qspi clock
3. introduce related macro definitions

Default QSPI is not enabled, since we need hardware rework to use QSPI,
see SPF-28590, page 9:
"
QSPI signals are muxed with EPDC_D[7:0]
When using QSPI: de-populate R388-R391, R396-R399
populate R392-R395, R299, R300
"

After hardware rework, define CONFIG_FSL_QSPI in mx7dsabresd.h. qspi
flashes can be deteced and read/erase/write. Log info:
"
=> sf probe
SF: Detected MX25L51235F with page size 256 Bytes, erase size 64 KiB, total 64 MiB
=> sf read 0x80000000 0 0x4000000
device 0 whole chip
SF: 67108864 bytes @ 0x0 Read: OK
=> sf erase 0 0x4000000
SF: 67108864 bytes @ 0x0 Erased: OK
=> sf write 0x80000000 0 0x4000000
device 0 whole chip
SF: 67108864 bytes @ 0x0 Written: OK
"

Cc: Fabio Estevam <fabio.estevam@freescale.com>
Cc: Adrian Alonso <aalonso@freescale.com>
Reviewed-by: Stefano Babic <sbabic@denx.de>
Reviewed-by: Jagan Teki <jteki@openedev.com>
Signed-off-by: Peng Fan <Peng.Fan@freescale.com>