PWM2 is commonly used to control voltage of PWM regulator of VDD_LOG in
RK3399. On the Firefly-RK3399 board, PWM2 outputs 40 KHz square wave
from power on and the VDD_LOG is about 0.9V. When the kernel boots
normally into the system, the PWM2 keeps outputing PWM signal.

But the kernel hangs randomly after "Starting kernel ..." line on that
board. When it happens, PWM2 outputs high level which causes VDD_LOG
drops to 0.4V below the normal operating voltage.

By adding "pclk_rkpwm_pmu" to the rk3399_pmucru_critical_clocks array,
PWM clock is ensured to be prepared at startup and the PWM2 output is
normal. After repeated tests, the early boot hang is gone.

This patch works on both Firefly-RK3399 and ROC-RK3399-PC boards.
Signed-off-by: Levin Du <djw@t-chip.com.cn>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>