V4L/DVB (12553): FM TX: si4713: Add Kconfig and Makefile entries

Simply add Makefile and Kconfig entries.

[hverkuil@xs4all.nl: auto-select I2C_SI4713]
Signed-off-by: Eduardo Valentin <eduardo.valentin@nokia.com>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>