The shift registers have an output pin that, when enabled, propagates
the values of its internal register to the pins. If another value comes
to the register while the output pin is disabled, this new value will
makae the older shift into the next register in the chain.

This patch adds support for daisy-chaining the registers, using the
regular SPI chip select mechanism to manage the output pin, and the
registers-number dt property to set the number of chained registers.
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>