MIPS: ath79: Introduce <dt-bindings/clock/ath79-clk.h>

The include/dt-bindings/clock/ath79-clk.h header file
is introduced so we can use symbolic identifiers for SoC clocks.

Signed-off-by: Antony Pavlov <antonynpavlov@gmail.com>
Cc: Gabor Juhos <juhosg@openwrt.org>
Cc: Alban Bedel <albeu@free.fr>
Cc: Michael Turquette <mturquette@baylibre.com>
Cc: Stephen Boyd <sboyd@codeaurora.org>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: linux-mips@linux-mips.org
Cc: linux-clk@vger.kernel.org
Cc: devicetree@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/12875/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>