lib: utils/timer: mtimer: only use regname for aclint

The parser will fail if the timer is clint timer and has regname
property. As the regname is only meaningful for aclint, it is more
robust to only check regname for aclint timer.

Fixes: 6112d584

 ("lib: utils/fdt: Allow to use reg-names when parsing ACLINT")
Signed-off-by: Inochi Amaoto <inochiama@outlook.com>
Reviewed-by: Anup Patel <anup@brainfault.org>