arm64: add a helper function to traverse arm64_ftr_regs

hulk inclusion
category: feature
bugzilla: 48052
CVE: NA

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If we want to emulate ID registers, we need to initialize ID registers
firstly.  This commit is to add a helper function to traverse
arm64_ftr_regs so that we can initialize ID registers from
arm64_ftr_regs.

Signed-off-by: zhanghailiang <zhang.zhanghailiang@huawei.com>
Signed-off-by: Peng Liang <liangpeng10@huawei.com>
Reviewed-by: Zhanghailiang <zhang.zhanghailiang@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>