powerpc/microwatt: Add litesd

This is the register layout of the litesd peripheral for the fusesoc
based Microwatt SoC.

It requires a description of the system clock, which is hardcoded to
100MHz.

Signed-off-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220930065012.2860577-1-joel@jms.id.au