The CLKOUTn may be fed from PLL1/2/3, but the PLL1/2/3 has to be enabled
first by setting PLL_CLKE bit 11 in CCM_ANALOG_SYS_PLLn_GEN_CTRL register.
The CCM_ANALOG_SYS_PLLn_GEN_CTRL bit 11 is modeled by plln_out clock. Fix
the clock tree and place the clkout1/2 under plln_sel instead of plain plln
to let the clock subsystem correctly control the bit 11 and enable the PLL
in case the CLKOUTn is supplied by PLL1/2/3.