dt-bindings: rtc: pcf2127: Reference spi-peripheral-props.yaml

dt-bindings: rtc: pcf2127: Reference spi-peripheral-props.yaml

PCF2127 is an SPI device, thus its binding should reference
spi-peripheral-props.yaml.

Add a reference to spi-peripheral-props.yaml to fix the following
dt-schema warning:

imx7d-flex-concentrator.dtb: rtc@0: 'spi-max-frequency' does not match any of the regexes: 'pinctrl-[0-9]+'

Signed-off-by: Fabio Estevam <festevam@denx.de>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://lore.kernel.org/r/20250317120356.2195670-1-festevam@gmail.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>