dt-bindings: Document common property for daisy-chained devices

dt-bindings: Document common property for daisy-chained devices

Many serially-attached GPIO and IIO devices are daisy-chainable.

    Examples for GPIO devices are Maxim MAX3191x and TI SN65HVS88x:
    https://datasheets.maximintegrated.com/en/ds/MAX31913.pdf
    http://www.ti.com/lit/ds/symlink/sn65hvs880.pdf

    Examples for IIO devices are TI DAC128S085 and TI DAC161S055:
    http://www.ti.com/lit/ds/symlink/dac128s085.pdf
    http://www.ti.com/lit/ds/symlink/dac161s055.pdf

We already have drivers for daisy-chainable devices in the tree but
their devicetree bindings are somewhat inconsistent and ill-named:

    The gpio-74x164.c driver uses "registers-number" to convey the
    number of devices in the daisy-chain.  (Sans vendor prefix,
    multiple vendors sell compatible versions of this chip.)

    The gpio-pisosr.c driver takes a different approach and calculates
    the number of devices in the daisy-chain by dividing the common
    "ngpios" property (Documentation/devicetree/bindings/gpio/gpio.txt)
    by 8 (which assumes that each chip has 8 inputs).

Let's standardize on a common "#daisy-chained-devices" property.
That name was chosen because it's the term most frequently used in
datasheets.  (A less frequently used synonym is "cascaded devices".)

Signed-off-by: Lukas Wunner <lukas@wunner.de>
Acked-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>