[media] Docbook: add entry IDs for enum fe_sec_voltage

enum fe_sec_voltage is documented together with FE_SET_VOLTAGE.

Add xrefs for each entry there. This makes the hyperlinks at
frontend.h to go directly to the right documentation.

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>
1 file changed