ARM: convert arm to arch_gettimeoffset()

Convert arm to use GENERIC_TIME via the arch_getoffset() infrastructure,
reducing the amount of arch specific code we need to maintain.

The arm architecture is the last arch that need to be converted.

Signed-off-by: John Stultz <johnstul@us.ibm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Cc: Andrew Morton <akpm@linux-foundation.org>