libsel4platsupport: add gpt_constants.h to timer.h to avoid plat specific includes at the top level

libsel4platsupport/mach_include/imx/sel4platsupport/mach/gpt.h

diff --git a/libsel4platsupport/mach_include/imx/sel4platsupport/mach/gpt.h b/libsel4platsupport/mach_include/imx/sel4platsupport/mach/gpt.h
index 7a23536..757b2d9 100644
--- a/libsel4platsupport/mach_include/imx/sel4platsupport/mach/gpt.h
+++ b/libsel4platsupport/mach_include/imx/sel4platsupport/mach/gpt.h
@@ -18,7 +18,7 @@
 
 #include <stdint.h>
 #include <sel4platsupport/timer.h>
-
+#include <platsupport/plat/gpt_constants.h>
 
 /**
  * Get an interface for an initialised gpt timer.