rtc: pcf2123: add proper compatible string

nxp,rtc-pcf2123 is not a proper compatible strong for this RTC. The part
name is only pcf2123 and is less confusing.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20190819182656.29744-9-alexandre.belloni@bootlin.com


Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>