There are only a few differences between PCF2127 and PCF2129 (PCF2127
has 512 bytes of general purpose SRAM and count-down timer).
The rtc-pcf2127 driver currently doesn't use the PCF2127 specific
functionality and Kconfig help text already says this driver supports
PCF2127/29, so we can simply add pcf2129 to device id list.