arm64: dts: salvator-x: add 12288000 for sound ADG

Current rcar_sound only has 11289600 (= for 44.1kHz) clock-frequency,
but it needs 12288000 for 48kHz too.
Otherwise, 48kHz based sound can't handle correctly.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>