rtc: pcf2127: fix a bug when not specify interrupts property

Fix a bug when not specify interrupts property in dts
as follows,
    rtc-pcf2127-i2c 1-0051: failed to request alarm irq
    rtc-pcf2127-i2c: probe of 1-0051 failed with error -22

Signed-off-by: Biwen Li <biwen.li@nxp.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20200915073213.12779-1-biwen.li@oss.nxp.com