Add nuvoton,spk-btl to configure the two loudspeaker outputs
as Bridge Tied Load

Cc: David Lin <CTLIN0@nuvoton.com>
Signed-off-by: Emanuele Ghidoli <emanuele.ghidoli@toradex.com>
Signed-off-by: Francesco Dolcini <francesco.dolcini@toradex.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20230104195350.149300-1-francesco@dolcini.itSigned-off-by: Mark Brown <broonie@kernel.org>