Implement RTC

[daniel5151]

The RTC device is described under section 20 in the ep93xx user guide.

A simple implementation might be to synchronize the RTCData register to the host machine's clock. A fancier implementation would add the ability to set the time as well (via RTCLoad). The RTC is also capable of generating interrupts, so there may have to be some plumbing there as well.

It's likely the device would take a similar approach to the timer device, where registers are lazilly updated when read / written to, while a background thread is responsible for generating interrupts.