arm64: defconfig: Enable LAN and BT support for RPi 3 B+

The Raspberry Pi 3 B+ has a Microchip LAN7515 (connect via USB) and
a Cypress CYW43455 (connect via UART). This patch enables the necessary
drivers.

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