Input: sun4i-lradc-keys - fix typo in binding documentation

Trivial typo fix ("mut" -> "must") in the sunxi LRADC-keys binding
documentation.

Signed-off-by: Karsten Merker <merker@debian.org>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>