riscv: virt: Use Goldfish RTC device

riscv: virt: Use Goldfish RTC device

We extend QEMU RISC-V virt machine by adding Goldfish RTC device
to it. This will allow Guest Linux to sync it's local date/time
with Host date/time via RTC device.

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