Summary: - I noticed that mtimer stops around 30min after boot. - Finally, I found that nesc overflows in riscv_mtimer_current(). - This commit fixes this issue. Impact: - None Testing: - Tested with rv-virt:nsh on QEMU-7.1 Signed-off-by: Masayuki Ishikawa <Masayuki.Ishikawa@jp.sony.com>