refactor(bl31): introduce vendor extend rodata section

The purpose of including vendor extend plat.ld.rodata.inc
linker script is for compactly collecting vendor rodata in
intrinsic rodata section.
If vendors define a standalone section and assign the section
placed after __RW_END__, the raw bindry(bl31.bin) will include
bss section with zero value and increase binary size.

Signed-off-by: Leon Chen <leon.chen@mediatek.com>
Change-Id: I46dd8b02bfb26af1dcca27f61b3ea29ca74bbbd6

bl31/bl31.ld.S

@@ -47,6 +47,10 @@ SECTIONS
         __RODATA_START__ = .;
         *(SORT_BY_ALIGNMENT(.rodata*))
 
+#if PLAT_EXTRA_RODATA_INCLUDES
+#include <plat.ld.rodata.inc>
+#endif
+
 	RODATA_COMMON
 
         /* Place pubsub sections for events */