ARM: at91: pm: fix at91rm9200 standby

[ Upstream commit 84e871660bebfddb9a62ebd6f19d02536e782f0a ]

at91rm9200 standby and suspend to ram has been broken since
00482a40. It is wrongly using AT91_BASE_SYS which is a physical address
and actually doesn't correspond to any register on at91rm9200.

Use the correct at91_ramc_base[0] instead.

Fixes: 00482a40 (ARM: at91: implement the standby function for pm/cpuidle)

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>