sunxi: SPL SPI: Add SPI boot support for the Allwinner R40 SoC

Now that we can easily select an SoC specific SPI0 base address, adding
support for the Allwinner R40 is fairly trivial:
We set the base address, add this SoC to the ones that use PC23 and
enable it in Kconfig.

This allows booting from SPI flash on R40 boards.

Tested on a Bananapi M2 Berry with SPI flash connected to the header pins.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>