net: ethernet: Add support for setting T1S PLCA parameters

The Zephyr's core ethernet code had to be adjusted to support setting T1S
PLCA parameters from user Zephyr programs.

Such approach allows more flexibility, as T1S network configuration;
especially PLCA node numbers, can be assigned not only via device tree
at compile time. For example user can read them from EEPROM and then
configure the network accordingly.

For now - the union in struct ethernet_t1s_param only consists of plca
structure. This can change in the future, when other T1S OA parameters -
like Receive/Transmit Cut-Through Enable (bits RXCTE/TXCTE in OA_CONFIG0
register) are made adjustable from user program.

Signed-off-by: Lukasz Majewski <lukma@denx.de>