powerpc/85xx: Add eSDHC support for MPC8569E-MDS boards

Note that eSDHC and DUART0 are mutually exclusive on MPC8569E-MDS
boards. Default option is DUART0, so eSDHC is disabled by default.
U-Boot will fixup device tree if eSDHC should be used instead of
DUART0.

Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>