dt-bindings: gpio: fairchild,74hc595: Document chip select vs. latch clock

From looking at the data sheets, it is not obvious that CS# and latch
clock can be treated at the same, but doing so works fine and saves the
hassle of (1) trying to specify a SPI device without CS, and (2) adding
another property to drive the latch clock[1].

[1]: https://lore.kernel.org/lkml/[email protected]/

Signed-off-by: J. Neuschäfer <[email protected]>
Reviewed-by: Linus Walleij <[email protected]>
Reviewed-by: Rob Herring (Arm) <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Bartosz Golaszewski <[email protected]>

Author: neuschaefer

Documentation/devicetree/bindings/gpio/fairchild,74hc595.yaml

@@ -6,6 +6,23 @@ $schema: http://devicetree.org/meta-schemas/core.yaml#
 
 title: Generic 8-bit shift register
 
+description: |
+  NOTE: These chips nominally don't have a chip select pin. They do however
+  have a rising-edge triggered latch clock (or storage register clock) pin,
+  which behaves like an active-low chip select.
+
+  After the bits are shifted into the shift register, CS# is driven high, which
+  the 74HC595 sees as a rising edge on the latch clock that results in a
+  transfer of the bits from the shift register to the storage register and thus
+  to the output pins.
+                      _   _       _   _
+  shift clock    ____| |_| |_..._| |_| |_________
+
+  latch clock                           * trigger
+                 ___                     ________
+  chip select#      |___________________|
+
+
 maintainers:
   - Maxime Ripard <[email protected]>