dt-bindings: arm, gic-v3: require that reserved cells are always 0

The arm,gic-v3 binding was written with good intentions and doesn't
enforce interrupt-cells to be 3, therefore making it easy to extend
the irq description in future if necessary:

  > Cells 4 and beyond are reserved for future use.

Unfortunately, this sentence is immediately followed up with:

  > When the 1st cell has a value of 0 or 1, cells 4 and beyond act as
  > padding, and may be ignored. It is recommended that padding cells
  > have a value of 0.

Consequently, any extensions to the PPI or SPI interrupt specifiers must
be able to work with random crap from legacy DTs, effectively
necessitating a new interrupt type in the first cell. Sigh.

This patch fixes the text so that additional, reserved cells are
required to be zero. This looks like a reasonable thing to require and
is already satisifed by the .dts files in-tree.

Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/interrupt-controller/arm,gic-v3.txt

@@ -24,9 +24,8 @@ Main node required properties:
 		1 = edge triggered
 		4 = level triggered
 
-  Cells 4 and beyond are reserved for future use. When the 1st cell
-  has a value of 0 or 1, cells 4 and beyond act as padding, and may be
-  ignored. It is recommended that padding cells have a value of 0.
+  Cells 4 and beyond are reserved for future use and must have a value
+  of 0 if present.
 
 - reg : Specifies base physical address(s) and size of the GIC
   registers, in the following order: