riscv/sifive: add SiFive FU540 PRCI driver

The SiFive FU540 Power Reset Clocking Interrupt block contains a PLL
that turns the input crystal (33.3MHz) into a 1-1.5GHz clock.
This clock in turn is divided by two to produce the tlclk, which is fed
into devices such as the SPI and I2C controllers.

Register a new clock device for the PRCI so that those devices can
read the correct clock through the clk framework.

Submitted by: kp
Sponsored by: Axiado