of: Introduce of_for_each_phandle() helper macro

With this macro any user can easily iterate over a list of
phandles. The patch also converts __of_parse_phandle_with_args()
to make use of the macro.

The of_count_phandle_with_args() function is not converted,
because the macro hides the return value of of_phandle_iterator_init(),
which is needed in there.

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