arm64: perf: fix event number mask

This patch fixes ARMV8_EVTYPE_* macros since evtCount (event number)
field width is 10bits in event selection register.

Signed-off-by: Vinayak Kale <vkale@apm.com>
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>