arch_timer: add error handling when the MPM global timer is cleared

Bug: 29000863
Signed-off-by: albert.zl_huang <albert.zl_huang@htc.com>
Change-Id: I2b5a28b0a9edb31bdaa1ca2310397dd2f36f6c23

Updated to use arch_timer_read_counter() as arch_counter_get_cntvct
doesn't exist in this kernel.

Signed-off-by: Chris Redpath <chris.redpath@arm.com>