net: stmmac: dwmac-stm32: Extract PMCR configuration

Pull the PMCR clock mux configuration into a separate function. This is
the final change of three, which moves external clock rate validation,
external clock selector decoding, and clock mux configuration into
separate functions. This should make the code easier to understand.
No functional change intended.

Signed-off-by: Marek Vasut <marex@denx.de>
Signed-off-by: Christophe Roullier <christophe.roullier@foss.st.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>