[media] smiapp: Call the clock "ext_clk"

As the clock framework makes it possible to assign a device specific name to
the clocks, remove the ability to use a named clock in the driver.

Signed-off-by: Sakari Ailus <sakari.ailus@iki.fi>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>