mmc: sunxi: Remove unnecessary pinmux option dependency

Now that the pinmux conflict is handled in the board code (by skipping
setup for the one conflicting MMC controller), the driver does not need
to be entirely disabled based on the UART pinmux.

Signed-off-by: Samuel Holland <samuel@sholland.org>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>