arm926ejs: 8-byte align stack to avoid LDRD/STRD problems

U-boot for Marvell Kirkwood boards no longer work after the EABI changes
introduced in commit f772acf8. This
turns out to be caused by a stack alignment issue. The armv5te
instructions ldrd/strd instructions require 8-byte alignment to work
properly (otherwise undefined behavior).

Tested on an OpenRD base board, where both printouts and ubifs stuff now
works.

Signed-off-by: Simon Kagstrom <simon.kagstrom@netinsight.net>