ARM: at91: pm: add macros for plla disable/enable

Add macros for PLLA disable and enable (in disable macro the PLLA
state will also be saved). This prepares the field for PLLA disable/enable
for suspend/resume on SAM9X60.

Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/1579522208-19523-4-git-send-email-claudiu.beznea@microchip.com