power: reset: at91-reset: fix power down register

In the case of at91sam9g45_restart(), the driver is writing
AT91_DDRSDRC_LPCB_POWER_DOWN to AT91_DDRSDRC_RTR, this should actually be
AT91_DDRSDRC_LPR.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Sebastian Reichel <sre@kernel.org>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>