Documentation: at91: Update Microchip SoC documentation

We move the former Atmel wording to the the new Microchip name for this SoC
family. With the name of the directory we also change the content
in relation with the update of the MAINTAINERS file.
The Datasheet links now point to real documents instead of 404s.

Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>