spi: Add Amlogic Meson SPI Flash Controller driver

The Amlogic Meson SoCs embeds a Flash oriented SPI Controller name SPIFC.
This driver, ported from the Linux meson-spi-spifc driver, add support
for this controller on the Amlogic Meson GX SoCs in U-Boot.

Tested-by: Jerome Brunet <jbrunet@baylibre.com>
Reviewed-by: Jagan Teki <jagan@openedev.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>