arm64/cpufeature: Use helper for ECV CNTPOFF cpufeature

The newly added support for ECV CNTPOFF open codes the recently added
helper ARM64_CPUID_FIELDS(), make use of the helper.  No functional
change.

Signed-off-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>
Link: https://lore.kernel.org/r/20230523-arm64-ecv-helper-v1-1-506dfb5fb199@kernel.org


Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>