Stop setting SDRC_POWER.PWDENA on boot.  There is a nasty erratum
(34xx erratum 1.150) that can cause memory corruption if PWDENA is
enabled.

Based originally on a patch from Samu P. Onkalo <samu.p.onkalo@nokia.com>.

Tested on BeagleBoard rev C2.
Signed-off-by: NPaul Walmsley <paul@pwsan.com>
Cc: Samu P. Onkalo <samu.p.onkalo@nokia.com>