dt-bindings: pinctrl: ralink: Fix 'enum' lists with duplicate entries

There's no reason to list the same value twice in an 'enum'. This was fixed
treewide in commit c3b00681

 ("dt-bindings: Fix 'enum' lists with
duplicate entries"), but this one got added in the merge window.

A meta-schema change will catch future cases.

Signed-off-by: Rob Herring <robh@kernel.org>
Acked-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Link: https://lore.kernel.org/r/20220606212239.1360877-1-robh@kernel.org