support for Amlogic SPI Flash Controller IP

Merge series from Xianwei Zhao <xianwei.zhao@amlogic.com>:

This Flash Controller is derived by adding an SPI path to the original
raw NAND controller. This controller supports two modes: raw mode and
SPI mode. The raw mode has already been implemented in the community
(drivers/mtd/nand/raw/meson_nand.c).
This submission supports the SPI mode.

Add the drivers and bindings corresponding to the SPI Flash Controller.