rtc: pcf2127: bugfix: watchdog build dependency

Disable watchdog registation when kernel is build without
watchdog functionality, and enable watchdog core otherwise.
This removes compile errors like the one below:

drivers/rtc/rtc-pcf2127.o: in function `pcf2127_probe.constprop.3':
rtc-pcf2127.c:(.text.unlikely+0x2c8): undefined reference to
`devm_watchdog_register_device'

Watchdog feature in chip will always be configured as
this is safe to do in both cases and minimize code churn.

Reported-by: Hulk Robot <hulkci@huawei.com>
Reported-by: YueHaibing <yuehaibing@huawei.com>
Fixes: bbc597561ce1 ("rtc: pcf2127: add watchdog feature support")
Signed-off-by: Bruno Thomsen <bruno.thomsen@gmail.com>
Link: https://lore.kernel.org/r/20190827143656.4734-1-bruno.thomsen@gmail.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>