EmbeddedPkg: import EfiTimeBaseLib (as TimeBaseLib)

Some generic RTC helper functions were broken out from the PL031
library for use with other RTCs in OpenPlatformPkg. Import the code back
here, realigning it with the current state of PL031RealTimeClockLib to
simplify comparisons.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Leif Lindholm <leif.lindholm@linaro.org>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>