ARM: at91: pm: do not disable/enable PLLA for ULP modes

There is no need to disable/enable PLLA when switching to one of the
ULP modes. The PLLA consumers should take care of this.

Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Ludovic Desroches <ludovic.desroches@microchip.com>