rtc: pcf8563: Fix the datasheet URL

The current datasheet URL is no longer valid.

Replace with a valid one.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20210603143447.2223353-3-festevam@gmail.com