hw: rtc: Add Goldfish RTC device

hw: rtc: Add Goldfish RTC device

This patch adds model for Google Goldfish virtual platform RTC device.

We will be adding Goldfish RTC device to the QEMU RISC-V virt machine
for providing real date-time to Guest Linux. The corresponding Linux
driver for Goldfish RTC device is already available in upstream Linux.

For now, VM migration support is available but untested for Goldfish RTC
device. It will be hardened in-future when we implement VM migration for
KVM RISC-V.

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