drivers: entropy: smartbond: Optimize driver PM

This commit should optimize the way the device is allowed
to enter the suspended state. Instead of returning a PM
error code to abort the PM process, the standby power state
is constrained as long as the device is not allowed to enter
suspension. With that approach, acquiring PD_SYS is not needed
when in PM device runtime mode.

Signed-off-by: Ioannis Karachalios <ioannis.karachalios.px@renesas.com>