arm64: Use register field helper in kaslr_requires_kpti()

Rather than open-code the extraction of the E0PD field from the MMFR2
register, we can use the cpuid_feature_extract_unsigned_field() helper
instead.

Acked-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>