dt-bindings: gpio-mmio: Add ngpios property

This adds the ngpios property to MMIO GPIO. We restrict the
property to 1..63 since there is no point in 0 GPIO lines and
we support up to 64bits wide registers for now.

Acked-by: Rob Herring (Arm) <[e-mail]>
Signed-off-by: Linus Walleij <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Bartosz Golaszewski <[e-mail]>

Documentation/devicetree/bindings/gpio/gpio-mmio.yaml

···
     description:
       A list of registers in the controller. The width of each register is
       determined by its size. All registers must have the same width. The number
-      of GPIOs is set by the width, with bit 0 corresponding to GPIO 0.
+      of GPIOs is set by the width, with bit 0 corresponding to GPIO 0, unless
+      the ngpios property further restricts the number of used lines.
     items:
       - description:
           Register to READ the value of the GPIO lines. If GPIO line is high,
···
         - dirin
 
   native-endian: true
+
+  ngpios:
+    minimum: 1
+    maximum: 63
+    description:
+      If this property is present the number of usable GPIO lines are restricted
+      to the first 0 .. ngpios lines. This is useful when the GPIO MMIO register
+      has 32 bits for GPIO but only the first 12 are actually connected to
+      real electronics, and then we set ngpios to 12.
 
   no-output:
     $ref: /schemas/types.yaml#/definitions/flag
···
       compatible = "brcm,bcm6345-gpio";
       reg-names = "dirout", "dat";
       reg = <0xfffe0406 2>, <0xfffe040a 2>;
+      ngpios = <15>;
       native-endian;
       gpio-controller;
       #gpio-cells = <2>;