Makefile: Support include files for .dts files

Linux supports this, and if we are to have compatible device tree files,
U-Boot should also.

Avoid giving the device tree files access to U-Boot's include/ directory.
Only include/dt-bindings is accessible.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Masahiro Yamada <yamada.m@jp.panasonic.com>

arch/arm/dts/include/dt-bindings

@@ -0,0 +1 @@
+../../../../include/dt-bindings

arch/microblaze/dts/include/dt-bindings

@@ -0,0 +1 @@
+../../../../include/dt-bindings

arch/sandbox/dts/include/dt-bindings

@@ -0,0 +1 @@
+../../../../include/dt-bindings

arch/x86/dts/include/dt-bindings

@@ -0,0 +1 @@
+../../../../include/dt-bindings

scripts/Makefile.lib

@@ -153,6 +153,7 @@ ld_flags       = $(LDFLAGS) $(ldflags-y)
 # Modified for U-Boot
 dtc_cpp_flags  = -Wp,-MD,$(depfile).pre.tmp -nostdinc                    \
 		 -I$(srctree)/arch/$(ARCH)/dts                           \
+		 -I$(srctree)/arch/$(ARCH)/dts/include                   \
 		 -undef -D__DTS__
 
 # Finds the multi-part object the current object will be linked into