ARM: dts: at91: sama7g5ek: set regulator voltages for standby state

Set regulator voltages for standby state to avoid wrong behavior of
system while in standby. The CPU voltage has been chosen as being the
one corresponding to OPP=600MHz. Next commit will set the 600MHz OPP
as the suspend OPP.

Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Link: https://lore.kernel.org/r/20220113144900.906370-8-claudiu.beznea@microchip.com