powerpc/83xx: Add eSDHC support for MPC837xE-RDB/WLAN boards

Actually, the support is already there, but it requires newer U-Boots
(to fill-in clock-frequency, and setup pin multiplexing).

Though, it appears that on RDB boards USBB pins aren't multiplexed
between USB and eSDHC (unlike MDS boards, where USB and eSDHC share
pctl and pwrfault pins).

So, for RDB boards we can safely setup pinmux and manually fill-in
clock-frequency, thus making eSDHC work even with older u-boots.

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