dt-bindings: rtc: qcom-pm8xxx-rtc: update register numbers

Extend registers up to 2, also document their names.

Also fixes warnings generated by `make qcom/sdm845-oneplus-fajita.dtb`:
arch/arm64/boot/dts/qcom/sdm845-oneplus-fajita.dt.yaml: rtc@6000: reg: [[24576], [24832]] is too long
        From schema: Documentation/devicetree/bindings/rtc/qcom-pm8xxx-rtc.yaml
arch/arm64/boot/dts/qcom/sdm845-oneplus-fajita.dt.yaml: rtc@6000: 'reg-names' does not match any of the regexes: 'pinctrl-[0-9]+'
        From schema: Documentation/devicetree/bindings/rtc/qcom-pm8xxx-rtc.yaml

Signed-off-by: David Heidelberg <david@ixit.cz>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20211213192946.111320-1-david@ixit.cz