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

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.com
Signed-off-by: Rob Herring <robh@kernel.org>