checkpatch: update $allowed_asm_includes macros, add reboot.h and time.h

Several architectures (e.g.  x86, MIPS, Blackfin) have asm/reboot.h and
asm/time.h header files, which are not included in linux/reboot.h and
linux/time.h headers.  This lead to generation of false positive errors.

Signed-off-by: Sergey Ryazanov <ryazanov.s.a@gmail.com>
Cc: Andy Whitcroft <apw@canonical.com>
Cc: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>