drivers/rtc/rtc-cmos.c: drivers/char/rtc.c features for DECstation support

This brings in drivers/char/rtc.c functionality required for DECstation
and, should the maintainers decide to switch, Alpha systems to use
rtc-cmos.

Specifically these features are made available:

* RTC iomem rather than x86/PCI port I/O mapping, controlled with the
  RTC_IOMAPPED macro as with the original driver.  The DS1287A chip in all
  DECstation systems is mapped in the host bus address space as a
  contiguous block of 64 32-bit words of which the least significant byte
  accesses the RTC chip for both reads and writes.  All the address and
  data window register accesses are made transparently by the chipset glue
  logic so that the device appears directly mapped on the host bus.

* A way to set the size of the address space explicitly with the
  newly-added `address_space' member of the platform part of the RTC
  device structure.  This avoids the unreliable heuristics that does not
  work in a setup where the RTC is not...