rtc: pcf2123: port to regmap

Also remove pcf2123_delay_trec. This claimed to add a 30ns delay to SPI
writes, but I could not see any reference to this requirement in the
datasheet. Closest thing I could find to a requirement are timings for the
SPI chip enable line, which cannot be controlled by this driver (the ndelay
came after the call to spi_write_then_read, which means it would sleep
after CE has already gone inactive). Things seem to work fine without it.

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