When writing the upper mtime, we should keep the original lower mtime
whose value is given by cpu_riscv_read_rtc() instead of
cpu_riscv_read_rtc_raw(). The same logic applies to writes to lower mtime.

Signed-off-by: Jason Chien <jason.chien@sifive.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20230728082502.26439-1-jason.chien@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>