mpc83xx: MPC837XERDB: Add support for FSL eSDHC

This patch adds support for eSDHC on MPC837XERDB boards. The WP
switch doesn't seem to work on RDB boards though, the WP pin is
always asserted (can see the pin state when it's in GPIO mode).

FSL DR USB and FSL eSDHC are mutually exclusive because of pins
multiplexing, so user should specify 'esdhc' or 'dr_usb' options
in the hwconfig environment variable to choose between the
devices.

p.s.
Now we're very close to a monitor len limit (196 bytes left using
gcc-4.2.0), so also increase the monitor len by one sector (64 KB).

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