dt-bindings: pwm: Add i.MX TPM PWM binding

Add i.MX TPM(Low Power Timer/Pulse Width Modulation Module) PWM binding.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>