spi: rockchip_sfc: add support for Rockchip SFC

This patch adds support for the Rockchip serial flash controller
found on the PX30 SoC. It should work for versions 3-5 of the SFC
IP, however I am only able to test it on v3.

This is adapted from the WIP SPI-MEM driver for the SFC on mainline
Linux. Note that the main difference between this and earlier versions
of the driver is that this one does not support DMA. In testing
the performance difference (performing a dual mode read on a 128Mb
chip) is negligible. DMA, if used, must also be disabled in SPL
mode when using A-TF anyway.

Signed-off-by: Chris Morgan <macromorgan@hotmail.com>
Signed-off-by: Jon Lin <jon.lin@rock-chips.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>