RISC-V: Select SYSCON Reboot and Poweroff for QEMU virt machine

The SYSCON Reboot and Poweroff drivers can be used on QEMU virt machine
to reboot or poweroff the system hence we select these drivers using
QEMU virt machine kconfig option.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Palmer Dabbelt <palmerdabbelt@google.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>