dt-bindings: media: rc: Fix redundant string

The linux,rc-map-name property is described using an enum, yet a value has
been put in that enum twice, resulting in a warning. Let's fix that.

Fixes: 7c31b9d6 ("media: dt-bindings: media: Add YAML schemas for the generic RC bindings")
Signed-off-by: Maxime Ripard <mripard@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>