nios2: add .gitignore entries for auto-generated files

Add .gitignore entries for nios2 specific files generated during the
build process.
Signed-off-by: NTobias Klauser <tklauser@distanz.ch>
Acked-by: NLey Foon Tan <ley.foon.tan@intel.com>

arch/nios2/boot/.gitignore

+*.dtb
+vmImage

arch/nios2/kernel/.gitignore

+vmlinux.lds