USB: ice40-hcd: Add pinctrl support

The bridge chip can operate in both SPI slave and master mode.
The chip select pin needs to be driven low during power-up to
use the bridge chip in SPI slave mode. The chip select pin is
controlled by the SPI controller driver. The configuration of
this pin needs to be overridden temporarily. The gpiomux
configuration has msm_gpiomux_write() API to achieve this. But
it requires accessing gpiomux_setting struct which will not
be available for 64-bit configuration. Hence remove the references
to the gpiomux. All the targets on which this driver is used are
supporting pinctrl framework. Use gpiolib API to control the
chip select pin during configuration loading.

Change-Id: I1172ff7f38461dce234e02526fb636159b22c4ad
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>