clk: bcm: rpi: Turn firmware clock on/off when preparing/unpreparing

clk: bcm: rpi: Turn firmware clock on/off when preparing/unpreparing

Currently, when we prepare or unprepare RPi's clocks, we don't actually
enable/disable the firmware clock. This means that
`clk_disable_unprepare()` doesn't actually change the clock state at
all, nor does it lowers the clock rate.

From the Mailbox Property Interface documentation [1], we can see that
we should use `RPI_FIRMWARE_SET_CLOCK_STATE` to set the clock state
off/on. Therefore, use `RPI_FIRMWARE_SET_CLOCK_STATE` to create a
prepare and an unprepare hook for RPi's firmware clock.

As now the clocks are actually turned off, some of them are now marked
CLK_IS_CRITICAL, as those are required to be on during the whole system
operation.

Link: https://github.com/raspberrypi/firmware/wiki/Mailbox-property-interface
Signed-off-by: Maíra Canal <mcanal@igalia.com>
Reviewed-by: Stefan Wahren <wahrenst@gmx.net>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>