arm64: add ARCH_HAS_OPP to allow enabling OPP library

The Operating Performance Point (OPP) Layer library is a generic
library used by CPUFREQ and DEVFREQ. It can be enabled only on the
platforms that specify ARCH_HAS_OPP option.

This patch selects that option in order to allow ARM64 based platforms
to use OPP library.

Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>