pinctrl: abx500: retire phandle config mechanism

The abx500 pin control driver supported a method of fetching the
generic config from a phandle to a separate node using the
"ste,config" as a phandle. This is not used in any device trees
and not documented in the bindings, so drop this support.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>