ARM: at91: remove at91sam9260/at91sam9g20 legacy board support

Second part of at91sam9260/at91sam9g20 legacy !DT removal. This is the core !DT
support removal for these two Atmel SoCs.
Use the Device Tree for running this board with newer kernels.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>