Grab timer frequency from FDT.

RISC-V timer has no dedicated DTS node and we have to get timer
frequency from cpus node.

Tested on Government Furnished Equipment (GFE) cores synthesized
on Xilinx VCU118.

Reviewed by: markj
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D19727