backlight: Delete the OT200 backlight driver

This driver has no in-kernel users. The device can only be populated
by board files since it does not support device tree nor ACPI,
and nothing in the kernel creates a device named "ot200-backlight".

This driver has been in the kernel since 2012. If it is used by
out-of-tree code that code should have been upstreamed by now,
it's been 8 years.

It uses the idiomatic forked GPIO of the CS5535 which combines
pin control and GPIO into its private custom interface, which
causes me a headache because that is not how we do things these
days: we creates separate pin control and GPIO drivers.

Delete this unused driver.

Cc: Christian Gmeiner <christian.gmeiner@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>