GD32: Use clocks instead rcu-periph-clock property

[nandojve]

Is your enhancement proposal related to a problem? Please describe.
The rcu-periph-clock property is not well align with devicetree and should be replaced. The correct property for that use is clocks.
This is a step closer to have a full clock capability on the GD32 SoCs family.

Describe the solution you'd like
Replace rcu-periph-clock property by correct clocks property. The change itself not require a implementation of clock driver to allow the fix.

Additional context
The solution can mimics the following implementation from Linux. A reference from &rcu node + RCU_TYPE_PERIPHERAL + clock identificator. The change to correct the property not seems to represent a high effort in the first look.

CC: @gmarull @cameled @soburi