auxdisplay: charlcd: make backlight initial state configurable

The charlcd driver currently flashes the backlight once on init.
This may not be desirable.  Thus, add options for turning the
backlight off or on as well.

Signed-off-by: Mans Rullgard <mans@mansr.com>
Signed-off-by: Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>