Enable SUN6I SPI controller for Allwinner ARM64 SoC's.

This would helpful to setup spi flash, for another booting source.
mark it as static since it require during boot.
Signed-off-by: NJagan Teki <jagan@amarulasolutions.com>
Signed-off-by: NMaxime Ripard <maxime.ripard@bootlin.com>