drivers: clocksource: arch_timer: enable user access to virt timers

Allow virtual timers i.e. CNTV_TVAL_EL0 to be accessed
by userspace.

CRs-Fixed: 1018301
Change-Id: I724ddbf4e7c02ee25622c6712210aee948d037f6
Signed-off-by: Kyle Yan <kyan@codeaurora.org>