[zircon][rtc] Fixes and some tests for rtclib

The new fallback driver needs to return sizeof(struct)
rather than ZX_OK. I was misled by the DDKTL.

Doing some cleanup and noticed no tests. Also made it
so there are less magic numbers.

TEST= unittest included. Also manual test on Pixelbook.

Change-Id: I5531d3888bae601179b4fcdd0949a3fdcdc6768f

[zircon][dev] Fallback RTC driver

This CL introduces a fake RTC driver which can be used on
any board which does not have an actual RTC chip.

The device does not save the last value set but even so
it is useful because the uper layers actually use
https://roughtime.googlesource.com/roughtime to set the
RTC at every boot, thus this device in that case is
as useful as a real RTC device.

TEST = this device is only bound on the Astro so boot
up to garnet in the astro connected to the
internet and via ssh issue $date and check is
correct.

Change-Id: I95aafb9e425305067fbdb2ffba130537d2ec551d

[rtc] share some common rtc code

- hoist some general-purpose code out of the intel-rtc driver
- call clock adjustment system call from vim2 driver

Test: verified ZX_CLOCK_UTC behaves correctly on a Vim2
Change-Id: I12577779826d99e392e70f29998647a2f111395f