ARM: dts: add Raspberry Pi 3 A+

The Raspberry Pi 3 A+ is similar to the Pi 3 B+ but has only 512 MB RAM,
1 USB 2.0 port and no Ethernet.

Compared to the Raspberry Pi 3 B it isn't possible to control BT_ON and
WL_ON separately.

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