RISC-V: Select Goldfish RTC driver for QEMU virt machine

We select Goldfish RTC driver using QEMU virt machine kconfig option
to access RTC device on QEMU virt machine.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Atish Patra <atish.patra@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>