clk: meson: g12a: mark fclk_div2 as critical

On Amlogic Meson G12b platform, similar to fclk_div3, the fclk_div2
seems to be necessary for the system to operate correctly as well.

Typically, the clock also gets chosen by the eMMC peripheral. This
probably masked the problem so far. However, when booting from a SD
card the clock seems to get disabled which leads to a system freeze.

Let's mark this clock as critical, fixing boot from SD card on G12b
platforms.

Fixes: 085a4ea9

 ("clk: meson: g12a: add peripheral clock controller")
Signed-off-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Tested-by: Anand Moon <linux.amoon@gmail.com>
Cc: Marek Szyprowski <m.szyprowski@samsung.com>
Link: https://lore.kernel.org/r/577e0129e8ee93972d92f13187ff4e4286182f67.1598629915.git.stefan@agner.ch