Add some simple tests of the watchdog timer in the always-on domain device
of HiFive 1 rev b.

Signed-off-by: Tommy Wu <tommy.wu@sifive.com>
Reviewed-by: Frank Chang <frank.chang@sifive.com>
Acked-by: Thomas Huth <thuth@redhat.com>
Acked-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20230627141216.3962299-4-tommy.wu@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>