V4L/DVB (6685): ir-keymaps.c: extra keys on winfast Y04G0033 remote

This change adds support for 4 extra keys on the remote currently being
shipped by leadtek with their "WinFast TV2000 XP/Expert" and
"WinFast PVR2000" cards. The remote P/N seems to be Y04G0033 and
you can see a picture of it here: http://lespinasse.org/y04g0033.jpg

The extra keys are at the bottom and are labeled MCE +VOL, -VOL, +CH, -CH.
I chose to map them to the F21-F24 keycodes, following the precedent of
ir_codes_gotview7135[], so as to differentiate these 'MCE' keys from the
other +VOL, -VOL, +CH, -CH 'arrow' keys higher up on the remote.

Signed-off-by: Michel Lespinasse <walken@zoy.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>