ARM: configs: at91: sama7: enable cpu idle

Enable CPU idle support for SAMA7 config.

Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Link: https://lore.kernel.org/r/20220113144900.906370-10-claudiu.beznea@microchip.com