scripts: Makefile.lib: prefer upstream DT includes with OF_UPSTREAM

Device trees imported from Linux (dts/upstream) include their
dt-bindings headers from dts/upstream/include, but the dtc preprocessor
search order places U-Boot's legacy in-tree include/ first. When a SoC
has both a legacy dt-bindings header (used by non-upstream in-tree
device trees) and an upstream one, the legacy copy shadows the upstream
one and silently misresolves the clock/reset/power macros whose
numbering changed between generations.

Prefer dts/upstream/include when building with CONFIG_OF_UPSTREAM. The
upstream imported device trees are written against the upstream
bindings, and no current OF_UPSTREAM user builds legacy in-tree device
trees.

scripts/Makefile.lib

@@ -255,6 +255,8 @@ upstream_dtsi_include = $(addprefix -I, $(srctree)/dts/upstream/src/ \
 		 $(sort $(dir $(wildcard $(srctree)/dts/upstream/src/arm64/*/*))) \
 		 $(sort $(dir $(wildcard $(srctree)/dts/upstream/src/arm64/*/*/*)))))
 dtc_cpp_flags  = -Wp,-MD,$(depfile).pre.tmp -nostdinc                    \
+		 $(if $(CONFIG_OF_UPSTREAM), \
+		      -I$(srctree)/dts/upstream/include) \
 		 $(UBOOTINCLUDE)                                         \
 		 -I$(dir $<)                                             \
 		 -I$(u_boot_dtsi_loc)                                     \