drivers: net: lan865x: Select NET_L2_ETHERNET_MGMT when LAN865x used

The NET_L2_ETHERNET_MGMT configuration option is required to allow
setting MAC address or PLCA parameters with the LAN865x driver.

To avoid mistakes with per-board configuration files - it has been moved
to Kconfig and automatically selected when the driver support is enabled.

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