drivers: timer: riscv_machine_timer: Use reg-names to access registers

This commit updates the riscv_machine_timer driver to resolve MTIME and
MTIMECMP register addresses by their `reg-names` instead of relying on
index order.

This improves clarity and robustness in DTS bindings, and is a prerequisite
for handling cases where not both MTIME and MTIMECMP registers are present
or accessible.

Signed-off-by: Chen Xingyu <hi@xingrz.me>
2 files changed