cmd: nand: Make the NAND options default to NAND_SUNXI

cmd: nand: Make the NAND options default to NAND_SUNXI

If we depend on the ARCH_SUNXI configuration option, the boards that do not
have NAND support enabled (with the associated options) will not compile
anymore.

Depend on the NAND driver configuration option to make sure that is not the
case.

Reported-by: Chen-Yu Tsai <wens@csie.org>
Tested-by: Chen-Yu Tsai <wens@csie.org>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>