ARM: pm: preserve r4 - r11 across a suspend

Make cpu_suspend()..return function preserve r4 to r11 across a suspend
cycle.  This is in preparation of relieving platform support code from
this task.

Acked-by: Frank Hofmann <frank.hofmann@tomtom.com>
Tested-by: Kevin Hilman <khilman@ti.com>
Acked-by: Jean Pihet <j-pihet@ti.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>