MIPS: ath79: export switch MDIO reference clock

On AR934x, the MDIO reference clock can be configured to a fixed 100 MHz
clock. If that feature is not used, it defaults to the main reference
clock, like on all other SoC.

Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: John Crispin <john@phrozen.org>
Acked-by: Rob Herring <robh@kernel.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