ARM: at91: remove at91sam9rl legacy board support

Remove legacy support for at91sam9rl boards.
This include board files removal plus all legacy code for non DT boards
support.
Use the Device Tree for running this board with newer kernels.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
[nicolas.ferre@atmel.com: Add sentence about DT]
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>