ASoC: sigmadsp: Split regmap and I2C support into separate modules

When the SigmaDSP module is built-in, but I2C is build as a module we'll get a
undefined reference:

	sound/built-in.o: In function `sigma_action_write_i2c':
		:(.text+0x5d8d4): undefined reference to `i2c_master_send'

This can happen if a audio driver that is using the regmap SigmaDSP interface is
built into the kernel, but I2C support is build as a module. To fix this split
the SigmaDSP module into three modules, one module providing the core
infrastructure and two small modules implementing the regmap and I2C interfaces.
This allows e.g. the core infrastructure and regmap support to be built into the
kernel while I2C support can still be build as a module.

Fixes: dab464b60 ("ASoC: Add ADAU1361/ADAU1761 audio CODEC support")
Reported-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>