perf: Change PMCR write to read-modify-write

Preserves the bitfields of PMCR(AArch32)/PMCR_EL0(AArch64)
during PMU reset. Reset routine should write a 1 to PMCR.C and PMCR.P
fields only to reset the counters. Other fields should not be changed
as they could be set before PMU initialization and their value must
be preserved even after reset.

Change-Id: I835bf41f89a8a4691e996e6766aed4b4c8ef4368
Acked-by: Abhiroop Basak <abasak@qti.qualcomm.com>
Signed-off-by: Prasad Sodagudi <psodagud@codeaurora.org>