ASoC: loongson: Use the `idma` identifier for internal DMA variables

ASoC: loongson: Use the `idma` identifier for internal DMA variables

The Loongson I2S controller can work with two types of DMA:
- Internal DMA (iDMA): integrated DMA engine, driven by dedicated
  registers and interrupts.
- External DMA (eDMA): generic DMA engine (e.g., dw_dmac), using the
  standard dmaengine API.

To distinguish these two distinct implementations, rename all
internal-DMA-related structures, functions, and the component driver
to use the "idma" prefix.

No functional change intended.

Signed-off-by: Binbin Zhou <zhoubinbin@loongson.cn>
Link: https://patch.msgid.link/58e91c54f2bf658ac9b773741ca2aebc3866e550.1780304703.git.zhoubinbin@loongson.cn
Signed-off-by: Mark Brown <broonie@kernel.org>