gpio: 74x164: add dt support for nxp's 74x594

The chip is also an 8 bit shift register which works out of the box as a GPO
expander with this patch

Signed-off-by: Nicolas Saenz Julienne <nicolassaenzj@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>