of: Change of_machine_is_compatible() to return bool

of_machine_is_compatible() currently returns a positive integer if it
finds a match. However none of the callers ever check the value, they
all treat it as a true/false.

So change of_machine_is_compatible() to return bool, which will allow
the implementation to be changed in a subsequent patch.

Suggested-by: Rob Herring <robh@kernel.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20231214103152.12269-2-mpe@ellerman.id.au