rtc: pcf2123: define registers and bit macros

Add defines for all 16 registers in the pcf2123.
Add defines for useful bits from several registers
I've tried to document all the registers, and
as best as possible, all the special bits they employ

Use BIT() wherever possible in the bit definitions

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