realtek: Add Lexra bus clock

realtek: Add Lexra bus clock

The CPU peripherals on RTL83xx/RTL930x are connected to the CPU via the
Lexra bus. This bus can provide a clock signal to these peripherals, but
no clock driver is currently available. Instead, use a fixed-clock to
provide the clock frequency, and update the dependent peripherals.

Lexra bus clock frequencies:
- RTL838x: 200MHz
- RTL839x: 200MHz
- RTL930x: 175MHz

Signed-off-by: Sander Vanheule <[email protected]>
Tested-by: Stijn Segers <[email protected]>
Tested-by: Paul Fertser <[email protected]>
Tested-by: Stijn Tintel <[email protected]>