checkpatch: reduce warnings for #include of asm/foo.h to check from arch/bar.c

It is much more likely that an architecture file will want to directly
include asm header files.  Reduce this WARNING to a CHECK when the
referencing file is in the arch directory.

Signed-off-by: Andy Whitcroft <apw@shadowen.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>