rpi: update documentation regarding RPi4

RPi4 is not yet supported in the official builds coming from the OP-TEE
project, update the table of supported RPi devices to reflect that.

Signed-off-by: Kunal Mukherjee <kunmukh@gmail.com>
Reviewed-by: Joakim Bech <joakim.bech@linaro.org>
diff --git a/building/devices/rpi3.rst b/building/devices/rpi3.rst
index 4a0b7a9..9edc78b 100644
--- a/building/devices/rpi3.rst
+++ b/building/devices/rpi3.rst
@@ -188,7 +188,9 @@
     +-------------------------------+------------+
     | Raspberry Pi 3 Model B        | Yes        |
     +-------------------------------+------------+
-    | Raspberry Pi 3+ Model B       | No         |
+    | Raspberry Pi 3+ Model B       | Yes        |
+    +-------------------------------+------------+
+    | Raspberry Pi 4                | No         |
     +-------------------------------+------------+
     | Zero - all versions           | No         |
     +-------------------------------+------------+