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