dt-bindings: rtc: pcf85363: Document pcf85263 real-time clock

The pcf85263 RTC is compatible with the pcf85363 RTC.

The difference between the pcf85263 and pcf85363 RTC is that the latter has
64 bytes more RAM. This renders them incompatible from a DT point of view.

Signed-off-by: Biju Das <biju.das@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>