SPL: SPI: select SPL_SPI_FLASH_SUPPORT on SPL_SPI_SUNXI

SPL: SPI: select SPL_SPI_FLASH_SUPPORT on SPL_SPI_SUNXI

The Allwinner SPI flash SPL boot support is guarded by the SPL_SPI_SUNXI
symbol. But despite its generic name, the actual only use case for this
is to provide SPI flash support to the SPL, which requires
CONFIG_SPL_SPI_FLASH_SUPPORT to be defined.
Select this symbol from the SPL_SPI_SUNXI Kconfig definition. This
avoids doing this explicitly in the defconfig, and fixes SPI booting on
the Pine64 SoPine (and -LTS version) and the OrangePi Win board (both with
SPI flash).

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>