hw/misc: sifive_e_aon: Support the watchdog timer of HiFive 1 rev b.

hw/misc: sifive_e_aon: Support the watchdog timer of HiFive 1 rev b.

The watchdog timer is in the always-on domain device of HiFive 1 rev b,
so this patch added the AON device to the sifive_e machine. This patch
only implemented the functionality of the watchdog timer.

Signed-off-by: Tommy Wu <tommy.wu@sifive.com>
Reviewed-by: Frank Chang <frank.chang@sifive.com>
Acked-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20230627141216.3962299-2-tommy.wu@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>