M68k generic RTC driver updates:
  - Revive help text for CONFIG_GEN_RTC
  - Re-add lost config option for CONFIG_GEN_RTC_X
  - Re-add lost mach_get_ss()
  - Export mach_[gs]et_rtc_pll()
  - Add implementation of mach_get_ss() and mach_[gs]et_rtc_pll() for Q40/Q60
  - Add missing include
  - Add implementation of get_rtc_ss()