For OProfile to initialise oprofilefs correctly, it needs to know
the number of counters it can represent.

This patch adds a function to the ARM perf-events backend to return
the number of hardware counters available for the current PMU.

Cc: Jamie Iles <jamie.iles@picochip.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>