dt-bindings: clock: mediatek: add binding for fixed-factor clock axisel_d4

Just add binding for a fixed-factor clock axisel_d4, which would be
referenced by PWM devices on MT7623 or MT2701 SoC.

Cc: stable@vger.kernel.org
Fixes: 1de9b216 ("clk: mediatek: Add dt-bindings for MT2701 clocks")
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: devicetree@vger.kernel.org
Signed-off-by: Stephen Boyd <sboyd@kernel.org>