dt-bindings: leds: cznic,turris-omnia-leds: fix error in binding

dt-bindings: leds: cznic,turris-omnia-leds: fix error in binding

There is a bug in the device tree binding for cznic,turris-omnia-leds
which causes make dt_binding_check to complain.

The reason is that the multi-led property binding's regular expression
does not contain the `@` character, while the example nodes do.

Fix this, and also adjust the maximum address to 'b' as there are 12
LEDs.

Cc: Rob Herring <robh+dt@kernel.org>
Cc: devicetree@vger.kernel.org
Cc: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Marek Behún <marek.behun@nic.cz>
Link: https://lore.kernel.org/r/20200915005426.15957-1-marek.behun@nic.cz
Signed-off-by: Rob Herring <robh@kernel.org>