[media] stv090x: added function to control GPIOs from the outside

This patch STV090X adds and exports a function to control the GPIOs of
the stv090x-devices.

Cc: Manu Abraham <abraham.manu@gmail.com>
Signed-off-by: Martin Wilks <m.wilks@technisat.com>
Signed-off-by: Patrick Boettcher <pboettcher@kernellabs.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>