ARM: at91: pm: Workaround DDRSDRC self-refresh bug with LPDDR1 memories.

As already explained for pm_suspend.S, the DDRSDR controller fails to put
LPDDR1 memories in self-refresh. Force the controller to think it has DDR2
memories during the self-refresh period, as the DDR2 self-refresh spec is
equivalent to LPDDR1, and is correctly implemented in the controller.

Acked-by: Wenyou Yang <wenyou.yang@atmel.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>