[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

[dev][rtc] basic implementation for VIM RTC

- ZX-1985

Test: 'clock' utility works as expected
Change-Id: I1646342bf8052ed275303a99affd07e0fde8ffcb