ARM: sunxi: remove bare default for CONFIG_MMC

The bare default entry is wrong.  Just remove it since the (real)
entry in drivers/mmc/Kconfig has "default ARM || PPC || SANDBOX".

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Marek Vasut <marex@denx.de>