[riscv] Read the timer frequency from the device tree

[Pedro Falcato]

Currently, lk relies on platform code to define ARCH_RISCV_MTIME_RATE with the timer's frequency.
This can be avoided by reading /cpus/timebase-frequency (for all CPUs, if it exists) and /cpus/cpu*/timebase-frequency (for each CPU) in the supplied FDT.