configs: add Raspberry Pi 3 defconfig

So far identical to the rpi2 one except for the dts file as the SW runs in
32bit mode.

[Peter: extend commit message]
Signed-off-by: Ubaldo Porcheddu <ubaldo@eja.it>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

board/raspberrypi/readme.txt

@@ -6,7 +6,8 @@ Intro
 These instructions apply to all models of the Raspberry Pi:
   - the original models A and B,
   - the "enhanced" models A+ and B+,
-  - the model B2 (aka Raspberry Pi 2).
+  - the model B2 (aka Raspberry Pi 2)
+  - the model B3 (aka Raspberry Pi 3).
 
 How to build it
 ===============
@@ -21,10 +22,14 @@ For models A, B, A+ or B+:
 
   $ make raspberrypi_defconfig
 
-And for model 2 B:
+For model 2 B:
 
   $ make raspberrypi2_defconfig
 
+For model 3 B:
+
+  $ make raspberrypi3_defconfig
+
 Build the rootfs
 ----------------
 
@@ -46,6 +51,7 @@ After building, you should obtain this tree:
     +-- bcm2708-rpi-b.dtb           [1]
     +-- bcm2708-rpi-b-plus.dtb      [1]
     +-- bcm2709-rpi-2-b.dtb         [1]
+    +-- bcm2710-rpi-3-b.dtb         [1]
     +-- boot.vfat
     +-- kernel-marked/zImage        [2]
     +-- rootfs.ext4