arm64: dts: apple: t8103: Rename clk24 to clkref

We now know that this frequency comes from the external reference
oscillator and is used for various SoC blocks, and isn't just a random
24MHz clock, so let's call it something more appropriate.

Reviewed-by: Mark Kettenis <kettenis@openbsd.org>
Reviewed-by: Sven Peter <sven@svenpeter.dev>
Signed-off-by: Hector Martin <marcan@marcan.st>