ARM: dts: OMAP5: Fix missing PWM capability to timer nodes

OMAP5 has 6 timers (GPTimers 5, 6, 8 to 11) that are capable of PWM.
The PWM capability property is missing from the node definitions of
couple of timers.

Add ti,timer-pwm attribute for timer 5, 6, 8 and 11.

Signed-off-by: Suman Anna <s-anna@ti.com>
[benoit.cousson@linaro.org: Update changelog and subject to highlight
the fix]
Signed-off-by: Benoit Cousson <benoit.cousson@linaro.org>