clk: meson: clean-up clk81 clocks

clk81 is a composite clock which parents all the peripheral clocks of the
platform. It is a critical clock which is used as provided by the
bootloader. We don't want to change its rate or reparent it, ever.

Remove the CLK_IGNORE_UNUSED on the mux and divider. These clock can't
gate so the flag is useless, and the gate is already critical, so the
clock won't ever be unused.

Remove CLK_SET_RATE_NO_REPARENT from mux, it is useless since the mux is
read-only.

Remove CLK_SET_RATE_PARENT from the gate and divider and use ro_ops for
the divider. A peripheral clock should not try to change the rate of
clk81. Stopping the rate propagation is good way to make sure such request
would be ignored.

Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>