pvmfw: Add swiotlb <reg> to platform.dts

Instead of inserting the property into the template DT (which requires
shifting all following nodes), have a placeholder property that can be
overwritten in place.

Ensure that it is removed (NOP-ed) when not required.

Test: m pvmfw_img
Change-Id: Ife13eb84e23c43655513dfedffdedb1ea7fd2e78
diff --git a/pvmfw/platform.dts b/pvmfw/platform.dts
index a7b1de7..74439d9 100644
--- a/pvmfw/platform.dts
+++ b/pvmfw/platform.dts
@@ -37,6 +37,7 @@
 		ranges;
 		swiotlb: restricted_dma_reserved {
 			compatible = "restricted-dma-pool";
+			reg = <PLACEHOLDER4>;
 			size = <PLACEHOLDER2>;
 			alignment = <PLACEHOLDER2>;
 		};