GPIO: gpio-dwapb: Enable platform driver binding to MFD driver

The Synopsys DesignWare APB GPIO driver only supports open firmware devices.
But, like Intel Quark X1000 SOC, which has a single PCI function exporting
a GPIO and an I2C controller, it is a Multifunction device. This patch is
to enable the current Synopsys DesignWare APB GPIO driver to support the
Multifunction device which exports the designware GPIO controller.

Reviewed-by: Hock Leong Kweh <hock.leong.kweh@intel.com>
Signed-off-by: Weike Chen <alvin.chen@intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>