clocksource: arch_timer: Enable user access to the physical counter

A user space application is planned to support feature for
synchronized timestamp among debug packets across peripherals.
As part of the feature, it is responsible for providing physical
timer count value to user space. So Enable user access to the
physical counter in cp15 register.

Change-Id: Idf7f6375713d842925e6f72a4b1fb98a7168726d
Signed-off-by: Se Wang (Patrick) Oh <sewango@codeaurora.org>