Add Goldfish RTC device driver for RISC-V

This device was originally used as part of the goldfish virtual hardware
platform used for emulating Android on QEMU, but is now also used as the
RTC for the RISC-V virt machine in QEMU. It provides a simple 64-bit
nanosecond timer exposed via a pair of memory-mapped 32-bit registers,
although only with 1s granularity.

Reviewed by: brooks (mentor), jhb (mentor), kp
Approved by: brooks (mentor), jhb (mentor), kp
Obtained from: CheriBSD
Differential Revision: https://reviews.freebsd.org/D25717