MIPS: ath79: add helpers for setting clocks and expose the ref clock

Preparation for transitioning the legacy clock setup code over
to OF.

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