soc: arm: st: set SWO reference frequency based on 'rcc'

Configure SWO reference frequency to be the same as
CONFIG_SYS_CLOCK_HW_CYCLES_PER_SEC. This should make all ST boards ready to
be used with SWO, without configuring any other devicetree property.

Tested with NUCLEO-L476RG and NUCLEO-H563ZI.

Signed-off-by: Marcin Niestroj <m.niestroj@emb.dev>