drivers: clock_control: Add initial SiM3U1xx support

This serves two main purposes:
- change the CPU clock via devicetree nodes
- provide the APB frequency to device drivers via the clock driver
  interface

Theoretically this could also support choosing between the available
clock sources, but right now we only support LPOSC0 going into PLL0,
going into AHB.

Turning the PLL back off is also not supported since the only current
use case is to set the PLL frequency, turn it on, and switch the AHB
over to it.

Signed-off-by: Michael Zimmermann <michael.zimmermann@grandcentrix.net>