ARM: at91: pm: do not panic if ram controllers are not enabled

In case PM is enabled but there is no RAM controller information
in DT the code will panic. Avoid such scenarios by not initializing
platform specific PM code in case RAM controller is not provided
via DT.

Reported-by: Eugen Hristev <eugen.hristev@microchip.com>
Fixes: 827de1f1 ("ARM: at91: remove at91_dt_initialize and machine init_early()")
Fixes: 892e1f4a

 ("ARM: at91: pm: add sama7g5 ddr phy controller")
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Link: https://lore.kernel.org/r/20210823131915.23857-2-claudiu.beznea@microchip.com