arm64: Remove stray ARCH_HAS_OPP reference

A reference to ARCH_HAS_OPP was added in commit 333d17e5 (arm64: add
ARCH_HAS_OPP to allow enabling OPP library) however this symbol is no
longer needed after commit 049d595a (PM / OPP: Make OPP invisible
to users in Kconfig).

Signed-off-by: Mark Brown <broonie@linaro.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>