ARM: OMAP: fix fault in enter_full_retention()

In omap24xx_cpu_suspend assembly routine, the r2 register which holds
the address of the SDRC_POWER reg is set to zero before the value is
written back triggering a fault due to writing to address zero.

It's hard to tell where this change was introduced since this file
has been moved and merged.

While this fix prevents a crash, suspend on my n810 is broken with
current kernels.  I never come out of suspend.

Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>