scripts/gdb: add hlist utilities

This allows easily examining kernel hlists in python.

Link: http://lkml.kernel.org/r/Message-ID:
Signed-off-by: Leonard Crestez <leonard.crestez@nxp.com>
Reviewed-by: Stephen Boyd <sboyd@kernel.org>
Cc: Jason Wessel <jason.wessel@windriver.com>
Cc: Jan Kiszka <jan.kiszka@siemens.com>
Cc: Kieran Bingham <kbingham@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>