ARM: shmobile: defconfig: Enable Ethernet AVB

The iWave RZ/G1M Q7 SOM supports Gigabit Ethernet Phy (Micrel KSZ9031MNX).
Gigabit Ethernet support is available in Renesas AVB driver.
To increase hardware support enable the driver in the shmobile_defconfig
multiplatform configuration.

Signed-off-by: Biju Das <biju.das@bp.renesas.com>
Reviewed-by: Chris Paterson <chris.paterson2@renesas.com>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>