drm/vc4: hdmi: Use reg-names to retrieve the HDMI audio registers

The register range used for audio setup in the previous generations of
SoC were always the second range in the device tree. However, now that
the BCM2711 has way more register ranges, it makes sense to retrieve it
by names for it, while preserving the id-based lookup as a fallback.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Tested-by: Chanwoo Choi <cw00.choi@samsung.com>
Tested-by: Hoegeun Kwon <hoegeun.kwon@samsung.com>
Tested-by: Stefan Wahren <stefan.wahren@i2se.com>
Link: https://patchwork.freedesktop.org/patch/msgid/a1ba5605fe1006a1ead5262ef3d66ea5d0750381.1599120059.git-series.maxime@cerno.tech