Add audio_clk_b configured as 0 Hz, this will be overridden by the
boards providing the audio clock.

Fixes: 8183a793 ("arm64: dts: renesas: r8a774e1: Add audio support")
Reported-by: Nobuhiro Iwamatsu <nobuhiro1.iwamatsu@toshiba.co.jp>
Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Link: https://lore.kernel.org/r/20201104105508.21197-1-prabhakar.mahadev-lad.rj@bp.renesas.comSigned-off-by: Geert Uytterhoeven <geert+renesas@glider.be>