clk: bcm: rpi: Manage clock rate in prepare/unprepare callbacks

clk: bcm: rpi: Manage clock rate in prepare/unprepare callbacks

On current firmware versions, RPI_FIRMWARE_SET_CLOCK_STATE doesn't
actually power off the clock. To achieve meaningful power savings, the
clock rate must be set to the minimum before disabling. This might be
fixed in future firmware releases.

Rather than pushing rate management to clock consumers, handle it
directly in the clock framework's prepare/unprepare callbacks. In
unprepare, set the rate to the minimum before disabling the clock.
In prepare, for clocks marked with `maximize` (currently v3d),
restore the rate to the maximum after enabling.

Signed-off-by: Maíra Canal <mcanal@igalia.com>
Reviewed-by: Maxime Ripard <mripard@kernel.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>