powerpc/83xx: Add FSL eSDHC support for MPC837x-RDB boards

Simply add appropriate sdhci nodes.

Note that U-Boot should configure pin multiplexing for eSDHC prior
to Linux could use it. U-Boot should also fill-in the clock-frequency
property (eSDHC clock depends on board-specific SCCR[ESDHCCM] bits).

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