ARM: AM43XX: Add functions to save/restore am43xx control registers

These registers are part of the wkup domain and are lost during RTC only
suspend and also hibernation, so storing/restoring their state is
necessary.

Signed-off-by: Tero Kristo <t-kristo@ti.com>
Signed-off-by: Keerthy <j-keerthy@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>