asm-generic: Add common asm-offsets.h

All architecture use same asm-offsets.h
So it generic header.
Signed-off-by: Yoshinori Sato <ysato@users.sourceforge.jp>
Acked-by: Arnd Bergmann <arnd@arndb.de>

include/asm-generic/asm-offsets.h

+#include <generated/asm-offsets.h>