rtc: pcf2127: add watchdog feature support

Add partial support for the watchdog functionality of
both PCF2127 and PCF2129 chips.

The programmable watchdog timer is currently using a fixed
clock source of 1Hz. This result in a selectable range of
1-255 seconds, which covers most embedded Linux use-cases.

Clock sources of 4096Hz, 64Hz and 1/60Hz is mostly useful
in MCU use-cases.

Countdown timer not available when using watchdog feature.

Signed-off-by: Bruno Thomsen <bruno.thomsen@gmail.com>
Acked-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20190822131936.18772-4-bruno.thomsen@gmail.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

drivers/rtc/Kconfig

@@ -888,7 +888,12 @@ config RTC_DRV_PCF2127
 	depends on RTC_I2C_AND_SPI
 	help
 	  If you say yes here you get support for the NXP PCF2127/29 RTC
-	  chips.
+	  chips with integrated quartz crystal for industrial applications.
+	  Both chips also have watchdog timer and tamper switch detection
+	  features.
+
+	  PCF2127 has an additional feature of 512 bytes battery backed
+	  memory that's accessible using nvmem interface.
 
 	  This driver can also be built as a module. If so, the module
 	  will be called rtc-pcf2127.