mmc: sunxi: revive depends on UART0_PORT_F

Commit f401e907 ("ARM: sunxi: remove bare default for
CONFIG_MMC") dropped "depends on UART0_PORT_F", but it is still
needed.  Revive it as a prerequisite of CONFIG_MMC_SUNXI.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>