rtc: pcf2123: clean up writes to the rtc chip

Add new functions pcf2123_write(), and pcf2123_write_reg().
Use named defines for the values being written.

This improves modularity and readability, and reduces lines of code.

Signed-off-by: Joshua Clayton <stillcompiling@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>