arm64: cpufeature: Add missing .field_width for system registers

hulk inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I8NRI6


CVE: NA

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This was missed when making specification of a field standard.

Fixes: 2ee4c87e ("arm64: cpufeature: Always specify and use a field width for capabilities")
Signed-off-by: Wang ShaoBo <bobo.shaobowang@huawei.com>