Adds ti,timer-pwm property to timers 4 to 7 to permit usage of their
PWM output fonctionnality via the dmtimer driver.

Cc: Brian Hutchinson <b.hutchman@gmail.com>
Signed-off-by: NNeil Armstrong <narmstrong@baylibre.com>
Signed-off-by: NTony Lindgren <tony@atomide.com>