drivers: net: lan865x: Move PLCA configuration to dedicated structure

This change allows modification of the PLCA configuration in the lan865x
driver.

Values in this structure can be set via device tree as well as modified
by the user program.

Without this change the latter use case would not be possible as the
struct lan865x_config structure is defined as read only and its data
is only provided by device tree.

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