ARM: at91: Documentation: add armv7m families

The Atmel sams70, samv70 and samv71 are Cortex-M7 based MCUs that can run
Linux (without MMU).

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