sunxi: Kconfig: Fix up SPI configuration

Commit 7945caf2 ("arm: sunxi: Enable SPI/SPI-FLASH support for A64")
selected CONFIG_SPI by default on all Allwinner A64 boards, even though
only 4 out of the 14 A64 boards have a SPI flash chip. All other SoCs
had to manually select DM_SPI and friends, even though they are a
platform property (the sunxi SPI driver is DM_SPI only).

Clean this up to allow easy selection of SPI flash support in U-Boot
proper, by selecting DM_SPI and DM_SPI_FLASH *if* CONFIG_SPI is
selected, for *all* Allwinner SoCs. This simplifies the defconfig for
two Libretech boards already.

Also remove the forced CONFIG_SPI from the A64 Kconfig, instead let the
four boards which allow SPI booting select this explicitly.

Any board wishing to support SPI flash in U-Boot proper now just defines
CONFIG_SPI and CONFIG_SPI_FLASH_<vendor> in its defconfig, Kconfig takes
care of the rest.

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