ARM: omap: fix warning with LPAE build

Some omap3 code is throwing a warning:
arch/arm/mach-omap2/pm34xx.c: In function 'omap3_save_secure_ram_context':
arch/arm/mach-omap2/pm34xx.c:123:32: warning: cast to pointer from
  integer of different size [-Wint-to-pointer-cast]

In reality this code will never actually execute with LPAE=y, since
Cortex-A8 doesn't support it. So downcasting the __pa() is safe in
this case.

Signed-off-by: Olof Johansson <olof@lixom.net>
Acked-by: Tony Lindgren <tony@atomide.com>