Add Goldfish RTC device driver for RISC-V

[jrtc27]

Summary

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.

[jrtc27]

Fixed stray non-breaking space in comment

[jrtc27]

... back to the right patch