ARM: at91: pm: fix SRAM allocation

On some platforms, there are multiple SRAM nodes defined in the device tree but
some of them are disabled, leading to allocation failure. Try to find the first
enabled SRAM node and allocate from it.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Tested-by: Wenyou Yang <wenyou.yang@atmel.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>