arm64/cpufeature: Use ARM64_CPUID_FIELD() to match EVT

The recently added Enhanced Virtualization Traps cpufeature does not use
the ARM64_CPUID_FIELDS() helper, convert it to do so. No functional
change.

Signed-off-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Zenghui Yu <zenghui.yu@linux.dev>
Link: https://lore.kernel.org/r/20230718-arm64-evt-cpuid-helper-v1-1-68375d1e6b92@kernel.org
Signed-off-by: Will Deacon <will@kernel.org>