drivers: clock_control: smartbond: Add USB clock

Smartbodn does not have dedicated USB clock.
For USB to work PLL needs to be turned on.
To allow for flexible configuration artificial USB clock
is added that can be operated via clock_control subsystem.
This new clock turns on PLL when USB subsystem is enabled.
PLL can also be request in DT if application requires
more speed.

PLL can be automatically turned off when USB enters suspend
state and application did not requested it.

Signed-off-by: Jerzy Kasenberg <jerzy.kasenberg@codecoup.pl>