[riscv] Add support for RDTIME as a timer source

[riscv] Add support for RDTIME as a timer source

The Zicntr extension defines an unprivileged wall-clock time CSR that
roughly matches the behaviour of an invariant TSC on x86.  The nominal
frequency of this timer may be read from the "timebase-frequency"
property of the CPU node in the device tree.

Add a timer source using RDTIME to provide implementations of udelay()
and currticks(), modelled on the existing RDTSC-based timer for x86.

Signed-off-by: Michael Brown <mcb30@ipxe.org>