dt-bindings: pwm: Use examples with documented/matching schema

'nvidia,tegra20-pwm' is not yet documented in the YAML schema. Use
'allwinner,sun7i-a20-pwm' instead to get rid of the following warning
with 'make DT_CHECKER_FLAGS=-m dt_binding_check':

	pwm/pwm.example.dt.yaml:0:0: /example-0/pwm@7000a000: failed to match
	any schema with compatible: ['nvidia,tegra20-pwm']
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Link: https://lore.kernel.org/r/20210608215834.2236920-1-sudeep.holla@arm.comSigned-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/pwm/pwm.yaml

@@ -24,8 +24,9 @@ additionalProperties: true
 
 examples:
   - |
-    pwm: pwm@7000a000 {
-        compatible = "nvidia,tegra20-pwm";
-        reg = <0x7000a000 0x100>;
-        #pwm-cells = <2>;
+    pwm: pwm@1c20e00 {
+        compatible = "allwinner,sun7i-a20-pwm";
+        reg = <0x01c20e00 0xc>;
+        clocks = <&osc24M>;
+        #pwm-cells = <3>;
     };