[media] ir: IR_RX51 only works on OMAP2

This driver can be enabled on OMAP1 at the moment, which breaks
allyesconfig for that platform. Let's mark it OMAP2PLUS-only
in Kconfig, since that is the only thing it builds on.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Timo Kokkonen <timo.t.kokkonen@iki.fi>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>