ARM: pm: avoid writing the auxillary control register for ARMv7

For ARMv7 kernels running in the non-secure world, writing to the
auxillary control register causes an abort, so we must avoid directly
writing the auxillary control register.  If the ACR has already been
reinitialized by SoC code, don't try to restore it.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>