MIPS: Alchemy: Add RTC device to all devboards

All Devboards can use the 32kHz counter as a RTC device.

Also delete the custom CMOS RTC header, which can be used for the
DS1693 on the PB1500.  But since it doesn't have a buffer battery
it is as useful as the on-chip RTC which I prefer.

Signed-off-by: Manuel Lauss <manuel.lauss@googlemail.com>
To: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/2874/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>