CRIS v32: Update driver for RTC chip pcf8563.

- Moved all calls to register_chrdev to a function called by module_init.
- Added mutex locking.
- Added better error handling at start up.
- Added BIN_TO_BCD of the month value before it is saved to the RTC.
- Corrected the month value returned by pcf8563_readreg.
- Cache the voltage low value at driver init so the battery status
  information does not get 'accidentally' cleared when setting the RTC time.
- Removed obsolete CONFIG_ETRAX_RTC_READONLY
- Voltage low ioctl():s RTC_VLOW_RD -> RTC_VL_READ, RTC_VLOW_SET -> RTC_VL_CLR