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

This reverts commit 2725d70a
("ARM: at91: pm: do not disable/enable PLLA for ULP modes").
This is because PLLA is the clock source for CPU, PLLA should
be disabled/enabled in the final/first phase of suspend/resume
so that the power consumption in suspend/resume to be minimal
and suspend/resume time to be minimized.

Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/1579522208-19523-3-git-send-email-claudiu.beznea@microchip.com