arm, arm926ejs: Flush the data cache before disabling it

The current implementation invalidates the data cache before turning it
off and causes problems on the hawkboard. See the discussion in
http://lists.denx.de/pipermail/u-boot/2012-January/115212.html



According to the ARM926EJ-S Technical Reference Manual, the cache should
be flushed instead.

Also fix the comments to match code.

Signed-off-by: Sughosh Ganu <urwithsughosh@gmail.com>

Rebased and corrected commit message.

Signed-off-by: Christian Riesch <christian.riesch@omicron.at>
Cc: Albert Aribaud <albert.u.boot@aribaud.net>
Cc: Tom Rini <trini@ti.com>
Acked-by: Heiko Schocher <hs@denx.de>
Tested-by: Heiko Schocher <hs@denx.de>