dt-bindings: pinctrl: Document pinconf bindings for BM1880 SoC

The pin configuration nodes act as a container for an arbitrary number of

subnodes. Each of these subnodes represents some desired configuration for a

pin, a group, or a list of pins or groups. This configuration for BM1880 SoC

includes only pinmux as there is no pinconf support available in SoC.

includes pinmux and various pin configuration parameters, such as pull-up,

slew rate etc...

Each configuration node can consist of multiple nodes describing the pinmux

options. The name of each subnode is not important; all subnodes should be

                  gpio66, gpio67, eth1, i2s0, i2s0_mclkin, i2s1, i2s1_mclkin,

                  spi0

Optional Properties:

- bias-disable:  No arguments. Disable pin bias.

- bias-pull-down: No arguments. The specified pins should be configured as

                  pull down.

- bias-pull-up:   No arguments. The specified pins should be configured as

                  pull up.

- input-schmitt-enable: No arguments: Enable schmitt trigger for the specified

                  pins

- input-schmitt-disable: No arguments: Disable schmitt trigger for the specified

                  pins

- slew-rate:      Integer. Sets slew rate for the specified pins.

                  Valid values are:

                  <0>  - Slow

                  <1>  - Fast

Example:

        pinctrl: pinctrl@400 {

                compatible = "bitmain,bm1880-pinctrl";