ARM: dts: sh73a0: Add missing clocks to sound node

The device node for the FIFO-buffered Serial Interface sound node lacks
the "clocks" property, as the DTS file didn't describe any clocks yet at
its introduction.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20200519075525.24742-1-geert+renesas@glider.be