ARM: at91: pm: add support for 2.5V LDO regulator control

Add support to disable/enable 2.5V LDO regulator when entering/exiting
any ULP mode.

Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Link: https://lore.kernel.org/r/20210415105010.569620-15-claudiu.beznea@microchip.com