ARM: dts: omap3-gta04: reduce panel backlight PWM frequency to 83Hz

This helps to get 100% intensity closer to "always on".

It compensates for an effect of dmtimer which at 100% still emits short
"off" impulses and the startup-time of the DC/DC converter makes
backlight intensity not reach full scale. The lower the PWM frequency
is, the smaller is this effect.

Signed-off-by: H. Nikolaus Schaller <hns@goldelico.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>