When building the modules 'modpost' warns about missing MODULE_DESCRIPTION.

Since almost none of the rc keymap modules have this, it produces a lot of
warnings.

As a first step to fixing all media modules, add this line to all keymaps.
The description should be a human-readable string describing the remote
or the remote controller that the keymap can be used with.

Note that keymaps/rc-cec.c is actually compiled into the rc-core, so that
is the sole keymap source that didn't need this.
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Sean Young <sean@mess.org>