ARM: at91: pm: move the setup of soc_pm.bu->suspended

Move the setup of soc_pm.bu->suspended in platform_suspend::begin
function so that the PMC code in charge with clocks suspend/resume
to differentiate b/w standard PM mode and backup mode.

Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Reviewed-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Link: https://lore.kernel.org/r/20210415105010.569620-3-claudiu.beznea@microchip.com