ARM: dts: at91: fix cpu node

The reg property is mandatory for cpu nodes, also fix the #address-cells of
the cpus node that has to be at least 1.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Reviewed-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Link: https://lore.kernel.org/r/20200831171129.3886857-2-alexandre.belloni@bootlin.com