renesas_rzg2l_smarc_defconfig: Enable networking support

renesas_rzg2l_smarc_defconfig: Enable networking support

For Ethernet to work on the RZ/G2L board, we need to enable support for
the ksz9131 PHY and enable random MAC address generation (as no MAC
address is programmed into the board).

We also enable the `dhcp`, `mii` and `ping` commands so that Ethernet
functionality can be tested and used to boot Linux.

Signed-off-by: Paul Barker <paul.barker.ct@bp.renesas.com>
Reviewed-by: Marek Vasut <marek.vasut+renesas@mailbox.org>