powerpc/85xx: Add eSDHC support for MPC8536DS boards

This patch simply adds sdhci node to the device tree.

We specify clock-frequency manually, so that eSDHC will work without
upgrading U-Boot. Though, that'll only work for default setup (1500
MHz) on new board revisions. For non-default setups, it's recommended
to upgrade U-Boot, since it will fixup clock-frequency automatically.

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