ARM: at91: Kconfig: select PM_OPP

Select PM_OPP. This is requested for CPUFreq driver.

Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Link: https://lore.kernel.org/r/20220113144900.906370-6-claudiu.beznea@microchip.com