soc: apple: Always include Makefile

We want to allow the code inside drivers/soc/apple to be compiled with
COMPILE_TEST but this will currently result in linking errors because
ARCH_APPLE is not set and make will never recurse into
drivers/soc/apple.
Let's just unconditionally recurse into apple/ since all drivers
in there are guarded by config options anyways.

Signed-off-by: Sven Peter <sven@svenpeter.dev>