rtc: mips: default to rtc-cmos on mips

The old rtc driver is getting in the way of some compat_ioctl
simplification. Looking up the loongson64 git history, it seems
that everyone uses the more modern but compatible RTC_CMOS driver
anyway, so let's remove the special case for loongson64.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>