System timer of ARMv8 runs at a different frequency than the CPU's.
The frequency is fixed, typically in the range 1-50MHz. It can be
read at CNTFRQ special register.
Change-Id: I6a21a6a9e2df783559df0caec63d5525c2258227 Signed-off-by: Gabriel Ganne <gabriel.ganne@enea.com>