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