ARM: dts: bcm2837: Add Raspberry Pi 3 B+

The Raspberry Pi 3 B+ has the following major differences compared
to the model 3 B:
* Microchip LAN7515 (Gigabit Ethernet with integrated USB 2.0 HUB)
* Cypress CYW43455 (802.11n/ac and BT 4.2)

We need to add the USB LAN chip so the bootloader can add the MAC address.
This is necessary because there ain't an EEPROM or a valid OTP.

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