The new HPET RTC emulation code uses a function in rtc.c, get_rtc_time().

So that function was made non-static.  Problem is, the same function name is
used elsewhere by some architectures, so there will be linkage problems.

And rtc_get_time() is used too, so I renamed it to rtc_get_rtc_time().

Also, the HPET code was setting CONFIG_HPET_EMULATE_RTC by hand in a header
file.  Fix it to use the Kconfig system properly.