MIPS: ath79: make specifying the reference clock in DT optional

It can be autodetected for many SoCs using the strapping options.
If the clock is specified in DT, the autodetected value is ignored

Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: John Crispin <john@phrozen.org>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: James Hogan <jhogan@kernel.org>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Pengutronix Kernel Team <kernel@pengutronix.de>
Cc: linux-mips@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: devicetree@vger.kernel.org