ARM: at91: pm: check for different controllers in at91_pm_modes_init()

at91_pm_modes_init() checks for proper nodes in device tree and maps
them accordingly. Up to SAMA7G5 all AT91 SoCs had the same mapping
b/w power saving modes and different controllers needed in the
final/first steps of suspend/resume. SAMA7G5 is not aligned with the
old SoCs thus the code is adapted for this. This patch prepares
the field for next commits.

Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Link: https://lore.kernel.org/r/20210415105010.569620-5-claudiu.beznea@microchip.com