The Denver CPUs implement support for PMUv3 for ARMv8.1 and expect the
PMCR_EL0 to be saved in non-secure context.

This patch disables cycle counter when event counting is prohibited
immediately on entering the secure world to avoid leaking useful
information about the PMU counters. The context saving code later
saves the value of PMCR_EL0 to the non-secure world context.

Verified with 'PMU Leakage' test suite.

 ******************************* Summary *******************************
 > Test suite 'PMU Leakage'
                                                                 Passed
 =================================
 Tests Skipped : 2
 Tests Passed  : 2
 Tests Failed  : 0
 Tests Crashed : 0
 Total tests   : 4
 =================================

Signed-off-by: Varun Wadekar <vwadekar@nvidia.com>
Change-Id: I3675e2b99b44ed23d86e29a5af1b496e80324875