ARM: at91/dt: add SRAM nodes

Add nodes for the SRAM available on atmel SoCs
For the at91sam9260 and the at91sam9g20, address mirroring is used to create a
single contiguous SRAM range instead of declaring two separate banks.

Also remove leftover TODOs in the sam9g45 file

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Boris Brezillon <boris.brezillon@free-electrons.com>
[nicolas.ferre@atmel.com: correct at91sam9rl sram size => 0x10000]
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>