dt-bindings: pwm: jz47xx: Remove unused compatible strings

Right now none of the Ingenic-based boards probe this driver from
devicetree. This driver defined three compatible strings for the exact
same behaviour. Before these strings are used, we can remove two of
them.
Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>

Documentation/devicetree/bindings/pwm/ingenic,jz47xx-pwm.txt

@@ -2,10 +2,7 @@ Ingenic JZ47xx PWM Controller
 =============================
 
 Required properties:
-- compatible: One of:
-  * "ingenic,jz4740-pwm"
-  * "ingenic,jz4770-pwm"
-  * "ingenic,jz4780-pwm"
+- compatible: Should be "ingenic,jz4740-pwm"
 - #pwm-cells: Should be 3. See pwm.txt in this directory for a description
   of the cells format.
 - clocks : phandle to the external clock.