V4L/DVB (3495): Kconfig: select VIDEO_CX25840 to build cx25840 a/v decoder module

The cx25840 module requires external firmware in order to function,
so it must select FW_LOADER, but saa7115 and saa7129 do not require it.
This patch creates VIDEO_CX25840, and alters VIDEO_DECODER to select it.

Signed-off-by: Michael Krufky <mkrufky@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>