platform.dts: Fix preprocessor bodging prop name

As Clang defines the C preprocessor macro `linux` as `1`, the following
property was incorrectly renamed in the preprocessed platform.dts:

  /chosen/linux,pci-probe-only --> /chosen/1,pci-probe-only

before being passed to DTC. The macro itself is <built-in>:

  error: 'linux' macro redefined [-Werror,-Wmacro-redefined]
     52 | #define linux
        |         ^
  <built-in>:474:9: note: previous definition is here
    474 | #define linux 1
        |         ^

Therefore, undefine it from the .dts, to prevent the substitution.

Bug: 376119598
Test: m pvmfw_platform.dts.preprocessed
Change-Id: I76e4e1c615bed55665089a6ab539a688fe4355e5
diff --git a/guest/pvmfw/platform.dts b/guest/pvmfw/platform.dts
index 44834ed..c3ecd0e 100644
--- a/guest/pvmfw/platform.dts
+++ b/guest/pvmfw/platform.dts
@@ -4,6 +4,11 @@
 
 #include <dt-bindings/interrupt-controller/arm-gic.h>
 
+// Undefine macros conflicting with our definitions.
+#ifdef linux
+#undef linux
+#endif
+
 #define PLACEHOLDER	0xffffffff
 #define PLACEHOLDER2	PLACEHOLDER PLACEHOLDER
 #define PLACEHOLDER4	PLACEHOLDER2 PLACEHOLDER2