clk: meson-gxbb: set fclk_div2 as CLK_IS_CRITICAL

clk: meson-gxbb: set fclk_div2 as CLK_IS_CRITICAL

On Amlogic Meson GXBB & GXL platforms, the SCPI Cortex-M4 Co-Processor
seems to be dependent on the FCLK_DIV2 to be operationnal.

The issue occurred since v4.17-rc1 by freezing the kernel boot when
the 'schedutil' cpufreq governor was selected as default :

  [   12.071837] scpi_protocol scpi: SCP Protocol 0.0 Firmware 0.0.0 version
  domain-0 init dvfs: 4
  [   12.087757] hctosys: unable to open rtc device (rtc0)
  [   12.087907] cfg80211: Loading compiled-in X.509 certificates for regulatory database
  [   12.102241] cfg80211: Loaded X.509 cert 'sforshee: 00b28ddf47aef9cea7'

But when disabling the MMC driver, the boot finished but cpufreq failed to
change the CPU frequency :

  [   12.153045] cpufreq: __target_index: Failed to change cpu frequency: -5

A bisect between v4.16 and v4.16-rc1 gave
05f814402d61 ("clk: meson: add fdiv clock gates") to be the first bad commit.
This commit added support for the missing clock gates before the fixed PLL
fixed dividers (FCLK_DIVx) and the clock framework basically disabled
all the unused fixed dividers, thus disabled a critical clock path for
the SCPI Co-Processor.

This patch simply sets the FCLK_DIV2 gate as critical to ensure
nobody can disable it.

Fixes: 05f814402d61 ("clk: meson: add fdiv clock gates")
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Tested-by: Kevin Hilman <khilman@baylibre.com>
[few corrections in the commit description]
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>