logging: swo: add Kconfig option for SWO reference frequency

SWO reference frequency was set based on `swo-ref-frequency` under `itm`
nodelabel or `/cpus/cpu@0/clock-frequency` property. Not all platforms
configure those.

All ST devices configure CPU frequency in `clock-frequency` under `rcc`
nodelabel. Configuring the same value for each board in
`/cpus/cpu@0/clock-frequency` would be one way to make SWO work out of the
box. There is lots of copy-pasting involved in this, which makes this very
error-prone.

Introduce Kconfig option, which will default to values configured in `itm`
or `/cpus/cpu@0`. The main advantage will be for platforms like ST, where
CPU clock frequency is already configured in another place. Thsoe could
override default value in SoC, board or any other platform specific layer.

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