MIPS: Allow platform to specify multiple its.S files

MIPS: Allow platform to specify multiple its.S files

In preparation for splitting arch/mips/generic/vmlinux.its.S into
multiple files such that it doesn't become a conflict magnet as boards
are added, allow platforms to specify a list of image tree source files
which will be concatenated to form the final source used to build the
image tree.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/16938/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>