On Amlogic Meson G12b platform, the fclk_div3 seems to be necessary for
the system to operate correctly.

Disabling it cause the entire system to freeze, including peripherals.

Let's mark this clock as critical, fixing boot on G12b platforms.
Signed-off-by: NNeil Armstrong <narmstrong@baylibre.com>
Signed-off-by: NJerome Brunet <jbrunet@baylibre.com>