overlays: Add PCF2129 RTC

Add support for the PCF2129 RTC to i2c-rtc and i2c-rtc-gpio overlays.
Also add rv3028 to i2c-rtc-gpio (it was missed previously), and don't
attempt to set an alternate address for the PCF2127.

Signed-off-by: Phil Elwell <phil@raspberrypi.org>