ASoC: ssm2518: Add explicit device tree support

Add OF match table to SSM2518 to allow direct matching without going
through I2C subsystem.

Signed-off-by: Ricard Wanderlof <ricardw@axis.com>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@kernel.org>