RPI: increase GPU memory to 128

Default GPU memory config is 64 MB, which was insufficient
for some of the B2Qt demos.

Task-number: QTEE-293
Change-Id: I6049c9577d1fea59619f07e9fc0ab4890f06a54b
Reviewed-by: Samuli Piippo <samuli.piippo@digia.com>

conf/distro/include/raspberrypi.conf

@@ -34,3 +34,6 @@ MACHINE_EXTRA_INSTALL_SDK = " \
         "
 
 module_autoload_snd-bcm2835 = "snd-bcm2835"
+
+# additional memory for GPU
+GPU_MEM = "128"