ARM: dts: dra7-l4: mark timer13-16 as pwm capable

DMTimers 13 - 16 are PWM capable and also can be used for CPTS input
signals generation. Hence, mark them as "ti,timer-pwm".

Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Reviewed-by: Lokesh Vutla <lokeshvutla@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>