powerpc/microwatt: Add LiteX MMC driver

Enable the LiteX MMC device and it's dependency the common clock
framework.

Signed-off-by: Joel Stanley <joel@jms.id.au>
Acked-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220804015708.5928-1-joel@jms.id.au