drivers: timer: get mtime cmp reg by reading mhartid

It is not guaranteed that a multi-core RISC-V hart numbering scheme
will match Zephyr's sequential cpu numbering scheme. Read the hartid and
use that value in calculation to get mtime_cmp reg, instead of the
current_cpu id.

Signed-off-by: Conor Paxton <conor.paxton@microchip.com>