ignore soc asm-offsets.s

Recent commit a4814a69 cleaned up generation of
asm-offsets.s for SoC dirs, but missed adding it to the ignore
list which makes it show up in `git status`.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>

.gitignore

@@ -46,6 +46,7 @@
 
 /include/generated/
 /lib/asm-offsets.s
+/arch/*/cpu/asm-offsets.s
 
 # stgit generated dirs
 patches-*