sunxi: Simplify MMC pinmux selection

Only one board, Yones Toptech BD1078, actually uses a non-default MMC
pinmux. All other uses of these symbols select the default value or an
invalid value. To simplify things, remove support for the unused pinmux
options, and convert the remaining option to a Boolean.

This allows the pinmux to be chosen by the preprocessor, instead of
having the code parse a string at runtime (for a build-time option!).
Not only does this reduce code size, but it also allows this Kconfig
option to be used in a table-driven DM pinctrl driver.

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>