[MIPS] Ignore vmlinux.lds generated files

This patch adds the proper .gitignore file to ignore vmlinux.lds generated
in arch/mips/kernel/.
Signed-off-by: NFlorian Fainelli <florian.fainelli@telecomint.eu>
Signed-off-by: NRalf Baechle <ralf@linux-mips.org>

arch/mips/kernel/.gitignore

+vmlinux.lds