ARM: at91: pm: avoid push and pop on stack while memory is in self-refersh

For the previous AT91 RAM controller and self-refresh procedure this
had no side effects. However, for SAMA7G5 the self-refresh procedure
doesn't allow this anymore as the RAM controller ports are closed
before switching it to self-refresh. This commits prepares the code
for the following ones adding self-refresh and PM support for SAMA7G5.
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Link: https://lore.kernel.org/r/20210415105010.569620-8-claudiu.beznea@microchip.com