panel-dt: Add handling of "sel6_8_polarity" property in device tree

Add support of optional "sel6_8_polarity" property to device tree, which defines the polarity of the "sel6_8" gpio pin. Because some panels (such as Kyocera TCG070WVLQ-AN*174) need "sel6_8" signal to be inverted for proper operation.

Signed-off-by: Dmitry Petrov dmitry.petrov@rtsoft.de

(cherry picked from commit 149f32f0)