gpio: revert bank bindings

Keep the words talking about what a GPIO bank is, but remove the
binding. We have not agreed that this is something we want to have.

Acked-by: Rob Herring <[e-mail]>
Signed-off-by: Linus Walleij <[e-mail]>

Documentation/devicetree/bindings/gpio/gpio.txt

···
 the fact that the hardware was synthesized by reusing the same IP block a
 few times over.
 
-A GPIO controller may specify a bank ID. This is a hardware index that
-indicate the logical order of the GPIO controller in the hardware architecture,
-usually in the sequence 0, 1, 2 .. n. The hardware index may be different
-from the order of register ranges and related to the backplane of how this
-one bank is connected to the outside through a pin controller for example.
-
 Optionally, a GPIO controller may have a "ngpios" property. This property
 indicates the number of in-use slots of available slots for GPIOs. The
 typical example is something like this: the hardware register is 32 bits
···
 	reg = <0x00000000 0x1000>;
 	gpio-controller;
 	#gpio-cells = <2>;
-	gpio-bank = <0>;
 	ngpios = <18>;
 }