arch/mips/dec: switch DECstation systems to rtc-cmos

This adds an RTC platform device for DECstation systems so that they can
use the rtc-cmos driver for their RTC device.

Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Cc: Ralf Baechle <ralf@linux-mips.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>