ARM: at91: Documentation: add sam9x60 product and datasheet

Add the new SAM9X60 ARM926-based SoC from Microchip and its associated
datasheet.

Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Link: https://lore.kernel.org/r/20200110090103.7728-1-nicolas.ferre@microchip.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>