rtc: pcf2127: Disable Power-On Reset Override

To resume normal operation after a total power loss (no or empty
battery) the "Power-On Reset Override (PORO)" facility needs to be
disabled.

The register reset value sets the PORO enabled and the data sheet
recommends setting it to disabled for normal operation.

From what I've seen on the PCF2127 and PCF2129 there is no event
generated at the interrupt pin (INT), as long the PORO bit is set. This
behavior is not documented in the manual.

Signed-off-by: Philipp Rosenberger <p.rosenberger@kunbus.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20210118085752.5759-2-p.rosenberger@kunbus.com