ARM: configs: at91: sama7: add config for cpufreq

Add config flags for CPUFreq. This includes enabling CPUFreq support,
CPUFreq DT driver and governors, default one being the conservative
governor.

Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Link: https://lore.kernel.org/r/20220113144900.906370-11-claudiu.beznea@microchip.com