lib: utils/timer: Simplify MTIMER synchronization

We simplify MTIMER synchronization as follows:

1) Detect MTIMER devices with unique (or non-shared) MTIME
   register at boot-time
2) Select first MTIMER device with no associated HART as our
   reference MTIMER device
3) Only synchronize MTIMER devices with unique (or non-shared)
   MTIME register using reference MTIMER device
4) Directly update the MTIME register at time of synchronization
   because MTIME is a read/write register.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>