kdump: update Documentation about crashkernel on arm64

hulk inclusion
category: feature
bugzilla: 12800
CVE: NA

-------------------------------------------------

Now we support crashkernel=X,[high,low] on arm64, update the
Documentation.

Signed-off-by: Chen Zhou <chenzhou10@huawei.com>
Reviewed-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>