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 <sander@svanheule.net>
Tested-by: Stijn Segers <foss@volatilesystems.org>
Tested-by: Paul Fertser <fercerpav@gmail.com>
Tested-by: Stijn Tintel <stijn@linux-ipv6.be>