video: hx8357: Make IM pins optional

The IM pins of the HX8357 controller are used to define the interface used to
feed pixel stream to the LCD panel.

Most of the time, these pins are directly routed to either the ground or the VCC
to set their values.

Remove the need to assign GPIOs to these pins when we are in such a case.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
fb: backlight: HX8357: Add HX8369 support

Add support for the Himax HX8369 controller as it is quite similar to the
hx8357.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>