ARM: dts: r8a7794: Use SYSC "always-on" PM Domain for sound

Hook up the Audio-DMAC and sound device nodes to the SYSC "always-on" PM
Domain, for a more consistent device-power-area description in DT.

Cfr. commit 0761ff2a ("ARM: dts: r8a7794: Add SYSC PM Domains").

Fixes: 320d6c5a ("ARM: dts: r8a7794: add sound support")
Fixes: 298e4ee3 ("ARM: dts: r8a7794: add Audio-DMAC support")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>