media: platform: Kconfig: place platform drivers on a submenu

Just like media bus drivers, place platform drivers on a
submenu, in order to better organize user-selection:

    Media drivers  --->
	    *** media drivers ***
	[*] Media USB Adapters  --->
	[*] Media PCI Adapters  --->
	-*- Radio Adapters  --->
	[*] Media platform devices  --->
	    *** MMC/SDIO DVB adapters ***
	< > Siano SMS1xxx based MDTV via SDIO interface
	[*] V4L test drivers  --->
	[*] DVB test drivers  --->
	    *** FireWire (IEEE 1394) Adapters ***
	<*> FireDTV and FloppyDTV
	    *** common driver options ***
	[ ] Enable Remote Controller support for Siano devices
	[ ] Enable debugfs for smsdvb

As this submenu depends on MEDIA_PLATFORM_DRIVERS and defaults to "y",
there's no need to change already-existing .config entries, nor touch
the several make *_defconfig.

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>