mfd: rn5t618: Add more subdevices

Since the RC5T619 has a RTC, use a separate subdevice list for that.
The ADC should be the same as in the RN5T618, according to drivers
in the wild, but since it is not tested, the ADC is only added for
the RC5T619.

Signed-off-by: Andreas Kemnade <andreas@kemnade.info>
Signed-off-by: Lee Jones <lee.jones@linaro.org>