MFC r302785, r308300, r308441

r302785 (jmcneill):
H3/A83T: Use PLL_PERIPH/2 for AHB2 parent clock.

Reviewed by: manu

r308300:
For AHB clock we need to set the assigned parents for cpufreq(4) to work.

r308441:
Do not fail to attach the clock if we cannot set the assigned parents as this
property isn't mandatory.