ASoC: mediatek: add the .probe() callback in mt2701_afe_pcm_dai_component

For the sake of uniformity, this patch adds a callback mt2701_afe_pcm_probe()
in mt2701_afe_pcm_dai_component to retrieve the regmap - the canonical way to
obtain the pointer..

Doing so, we could switch to use devm_snd_soc_register_component() to register
the component driver.

Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Mark Brown <broonie@kernel.org>