ion: remove use of qcom,msm-ion-reserve

The device tree string "qcom,msm-ion-reserve" is no
longer needed since we don't support carveout heaps any more.

Change-Id: Icb24d86fff9b478504d17518fda4c2e7b04833b8
Signed-off-by: Larry Bassel <lbassel@codeaurora.org>