arm64: cpufeature: Add missing .field_width for GIC system registers

This was missed when making specification of a field standard.

Fixes: 0a2eec83 ("arm64: cpufeature: Always specify and use a field width for capabilities")
Reported-by: Qian Cai <quic_qiancai@quicinc.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20220302134225.159217-1-broonie@kernel.org


Signed-off-by: Will Deacon <will@kernel.org>