nios2: add .gitignore entries for auto-generated files

Add .gitignore entries for nios2 specific files generated during the
build process.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Acked-by: Ley Foon Tan <ley.foon.tan@intel.com>

arch/nios2/boot/.gitignore

@@ -0,0 +1,2 @@
+*.dtb
+vmImage

arch/nios2/kernel/.gitignore

@@ -0,0 +1 @@
+vmlinux.lds