ARM: at91: pm: introduce at91_soc_pm structure

To have per SoC PM information add a new structure which embed
a member of type struct at91_pm_data. This will allow easy addition
of new information without contaminate struct at91_pm_data that is
passed to the last phase suspend function (at91_suspend_sram_fn).

Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Ludovic Desroches <ludovic.desroches@microchip.com>