[media] si2157: advertise Si2158 A20 firmware

Driver uses that firmware. Add it module firmware list.

Signed-off-by: Antti Palosaari <crope@iki.fi>
Tested-by: Olli Salonen <olli.salonen@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>