arm64: Add CNTFRQ_EL0 cp15 AArch32 trap handler

As cntfrq and cntvct share the same control bit, cntfrq_el0
access was disabled by default, in
'commit 7b4edf24 ("clocksource: arch_timer: make virtual
counter access configurable")'. There is a trap handler added
for user access using mrs instruction in
'commit e9632ce8 ("arm64: Add CNTFRQ_EL0 trap handler")'.
However, el0 running in AArch32 can try to access cntfrq
using mrc instruction. So, add a trap handler to support that.

Change-Id: I60d65b73961f3e35377a5601b7bfe5db37947f68
Signed-off-by: Neeraj Upadhyay <neeraju@codeaurora.org>