ARM: dts: Reduce qseecom heap sizes on sdxprairie

sdxprairie is a low memory target. This change reduces
the heap size used by qseecom so that it can be available
for other modules.

Change-Id: I0a87bb9fd0ab3a2a9f2eba939b0f97d805f008ee
Signed-off-by: Anmolpreet Kaur <anmolpre@codeaurora.org>