arm64: add a helper function to traverse arm64_ftr_regs

hulk inclusion
category: feature
bugzilla: NA
CVE: NA

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>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>