atmel: add support for at91sam9g20ek to flasher.sh

Adds support for the at91sam9g20-ek board to the flasher.sh script.

Signed-off-by: Prasanthi Chellakumar <prasanthi.chellakumar@microchip.com>
Signed-off-by: Joshua Henderson <joshua.henderson@microchip.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>