It allows a selectable timer interrupt frequency of 100, 250, 300 and 1000 HZ.
We will get better performance when choose a suitable frequency in some scene.

Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: NKefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: NCatalin Marinas <catalin.marinas@arm.com>