mpc83xx: MPC837XEMDS: Fixup eSDHC nodes in device tree

fdt_fixup_esdhc() will either disable or enable eSDHC nodes, and
also will fixup clock-frequency property.

Plus, since DR USB and eSDHC are mutually exclusive, we should
only configure the eSDHC if asked through hwconfig.

Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Acked-by: Kim Phillips <kim.phillips@freescale.com>