ASoC: dt-bindings: clarify areg-supply documentation

ASoC: dt-bindings: clarify areg-supply documentation

The documentation for areg-supply could cause confusion mainly in terms
of the relationship between AREG and AVDD.
According to the datasheet[1] the AREG can be one of two cases:

1) an external 1.8V supply
2) generated by an internal regulator (hence a 1.8V output)

[1] https://www.ti.com/lit/ds/symlink/tlv320adc5140.pdf

Signed-off-by: Emil-Juhl <juhl.emildahl@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@oss.qualcomm.com>
Link: https://patch.msgid.link/20260113-sound-soc-codecs-tvl320adcx140-v4-7-8f7ecec525c8@pengutronix.de
Signed-off-by: Mark Brown <broonie@kernel.org>