avr32: pm_standby low-power ram bug fix

The value stored into the SDRAMC LPR register should be the current
value of the register with the Self-refresh value set in the lower bit
field.

The bug involved only the Self-refresh value being written to the
register, thus over writing any low-power ram settings.

Signed-off-by: Humphrey Bucknell <hbucknell@saitek.com>
Signed-off-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>