ARM: at91: pm: Simplify at91rm9200_standby

Since 2008, AT91_MC_SDRAMC_LPR is set to 0 at kernel initialization. There
is no use saving, changing and restoring it.

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