of: Remove counting special case from __of_parse_phandle_with_args()

The index = -1 case in __of_parse_phandle_with_args() is
used to just return the number of phandles. That special
case needs extra handling, so move it to the place where it
is needed: of_count_phandle_with_args().

This allows to further simplify __of_parse_phandle_with_args()
later on.

Signed-off-by: Joerg Roedel <jroedel@suse.de>
Signed-off-by: Rob Herring <robh@kernel.org>