pinctrl: zynq: configure SPI SSx pins separately

Since SCLK, MISO and MOSI are the only mandatory signals at Zynq's SPI
interfaces, SS0, SS1 and SS2 have to be configured separately as they may
be used as simple GPIO lines.

This, of course, has to be considered in the devicetree, so pin controller
configuration for e.g. an SPI0 using SS0 and SS1 only might look like the
following snippet (derived from the example of chapter "17.5.3
MIO/EMIO" Routing of Zynq-7000 TRM UG585). So MIO20 can now be used
as GPIO instead of being occupied by SPI0 SS2 function. Note the separate
pinmux function for the slave select signals:

pinctrl_spi0_default: spi0-default {
	mux_spi {
		function = "spi0";
		groups = "spi0_0_grp";
	};

	mux_ss {
		function = "spi0_ss";
		groups = "spi0_0_ss0_grp", "spi0_0_ss1_grp";
	}

	conf-output {
		pins = "MIO16", "MIO21";
		slew-rate = <0>;
		bias-disable;
		low-power-disable;
		io-standard = <1>;
	};

	conf-input {
		pins = "MIO17";
		slew-rate = <0>;
		bias-high-impedance;
		low-power-disable;
		io-standard = <1>;
	};

	conf-select {
		pins = "MIO18", "MIO19";
		slew-rate = <0>;
		bias-pull-up;
		low-power-disable;
		io-standard = <1>;
	};
};

pinctrl_gpio0_default {
	mux {
		function = "gpio0";
		groups = "gpio0_20_grp"
	};

	conf {
		pins = "MIO20";
		slew-rate = <0>;
		bias-pull-up;
		low-power-disable;
		io-standard = <1>;
	};
};

Signed-off-by: Helmut Buchsbaum <helmut.buchsbaum@gmail.com>
Acked-by: Sören Brinkmann <soren.brinkmann@xilinx.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>