ARM: mcpm, perf/arm-cci: export mcpm_is_available

Now that the ARM CCI PMU driver can be built as a loadable module,
we get a link failure when MCPM is enabled:

ERROR: "mcpm_is_available" [drivers/perf/arm-cci.ko] undefined!

The simplest fix is to export that helper function.

Fixes: 8b0c93c2

 ("perf/arm-cci: Allow building as a module")
Acked-by: Nicolas Pitre <nico@linaro.org>
Acked-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Will Deacon <will.deacon@arm.com>