ARM: bcm2835_defconfig: Enable LAN78XX driver

This patch enables the necessary driver for Raspberry Pi 3 B+,
which has a LAN7515 chip.

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Eric Anholt <eric@anholt.net>