ARM: at91: pm: add plla disable/enable support for sam9x60

Add PLLA enable/disable support for 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/1579522208-19523-8-git-send-email-claudiu.beznea@microchip.com