clk: mediatek: fix PWM clock source by adding a fixed-factor clock

The clock for which all PWM devices on MT7623 or MT2701 actually depending
on has to be divided by four from its parent clock axi_sel in the clock
path prior to PWM devices.

Consequently, adding a fixed-factor clock axisel_d4 as one-fourth of
clock axi_sel allows that PWM devices can have the correct resolution
calculation.

Cc: stable@vger.kernel.org
Fixes: e9862118 ("clk: mediatek: Add MT2701 clock support")
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>