net: stmmac: dwmac-stm32: Mask support for PMCR configuration

Add possibility to have second argument in syscon property to manage
mask. This mask will be used to address right BITFIELDS of PMCR register.

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