test: posix: nanosleep: use k_cycle_64 as possible

as posix nanosleep is actually call us, so we need use a
higher clock to compare the result. try to use k_cycle_64
as possible.

Co-authored-by: Christopher Friedt <cfriedt@meta.com>
Signed-off-by: Hake Huang <hake.huang@oss.nxp.com>