Accept partial 'gpio-line-names' property.

Documentation/devicetree/bindings/gpio/gpio.txt says: "The names are
assigned starting from line offset 0 from left to right from the
passed array. An incomplete array (where the number of passed named
are less than ngpios) will still be used up until the last provided
valid line index". This patch makes it actually work this way.

Signed-off-by: Christophe Blaess <christophe.blaess@logilin.fr>
Signed-off-by: Patrick Boettcher <p@yai.se>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>