sunxi: Skip MMC0 init when its pinmux conflicts with UART0

sunxi: Skip MMC0 init when its pinmux conflicts with UART0

Currently, selecting UART0_PORT_F entirely disables MMC support on sunxi
platforms. But this is a bigger hammer then needed. Muxing UART0 to the
pins on port F only causes a conflict with MMC0, so minimize the impact
by specifically skipping MMC0 init. We can continue to use MMC1/2 if
those are enabled.

Let's also remove the preprocessor check while refacting this function.

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