rtc: pcf8523: switch to regmap

Use regmap to access the RTC registers, this is a huge reduction in code
lines and generated code. Values on ARMv7:

   text	   data	    bss	    dec	    hex
   5180	    132	      0	   5312	   14c0	before
   3900	    132	      0	   4032	    fc0	after

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20211018153651.82069-1-alexandre.belloni@bootlin.com