arm: perf: add Cortex-A15 PMU in armv7_pmu_probe_table

A15 PMU support was added by commit 14abd038 ("ARM: perf:
add support for the Cortex-A15 PMU"), but when the ARM perf
code was refactored in commit 29ba0f37 ("arm: perf: factor
out armv7 pmu driver"), A15 PMU was not included in
armv7_pmu_probe_table[]. This commit adds it.

Signed-off-by: John Jacques <john.jacques@intel.com>