lib: utils/timer: Add ACLINT MTIMER library

We add common ACLINT MTIMER library similar to the CLINT library
so that OpenSBI platforms can use it.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Xiang W <wxjstz@126.com>