MIPS: DEC: Ensure RTC platform device deregistration upon failure

MIPS: DEC: Ensure RTC platform device deregistration upon failure

Switch RTC platform device registration from platform_device_register()
to platform_add_devices() so as to make sure any failure will result in
automatic device unregistration.

Fixes: fae67ad43114 ("arch/mips/dec: switch DECstation systems to rtc-cmos")
Signed-off-by: Maciej W. Rozycki <macro@orcam.me.uk>
Acked-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>