board/boundarydevices: update readme.txt for nitrogen8mp

Add mention about nitrogen8mp_defconfig which is meant for our
Nitrogen8MPlus SOM device.

Signed-off-by: Gary Bisson <gary.bisson@boundarydevices.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

board/boundarydevices/common/readme.txt

@@ -31,6 +31,9 @@ Here is the list of targeted platforms per defconfig:
   - Nitrogen8MNano
   - Nitrogen8MNano_SOM
 
+- nitrogen8mp_defconfig
+  - Nitrogen8MPlus_SOM
+
 To install, simply copy the image to your storage (SD, eMMC, USB):
 
 $ sudo dd if=output/images/sdcard.img of=/dev/sdX