[media] em28xx: improve em2820-em2873/83 GPIO port register definitions and descriptions

- add definition for GPIO register 0x09 (reading/input)
- extend the information the chip variants that support GPIO registers 0x08/0x09
- rename EM28XX_R08_GPIO to EM2820_R08_GPIO_CTRL

Signed-off-by: Frank Schäfer <fschaefer.oss@googlemail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>