media: si470x-i2c: Add optional reset-gpio support

If reset-gpio is defined, use it to bring device out of reset.
Without this, it's not possible to access si470x registers.

Signed-off-by: Pawe? Chmiel <pawel.mikolaj.chmiel@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>