ARM: dts: mmp2: Use symbolic names for audio clocks

These are slighly easier to read.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Link: https://lore.kernel.org/r/20210121034130.1381872-8-lkundrak@v3.sk

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Signed-off-by: Arnd Bergmann <arnd@arndb.de>