ARM: at91: remove at91sam9g45/9m10 legacy board support

Remove legacy support for at91sam9g45/9m10 boards.
This include board files removal plus all legacy code for non DT boards
support (i.e. at91sam9g45.c and at91sam9g45_devices.c).
Use the Device Tree for running this board with newer kernels.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Acked-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
[nicolas.ferre@atmel.com: Add sentence about DT, removed defconfig]
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>