eal/armv8: use high-resolution cycle counter

eal/armv8: use high-resolution cycle counter

Existing cntvct_el0 based rte_rdtsc() provides portable
means to get wall clock counter at user space. Typically
it runs at <= 100MHz.

The alternative method to enable rte_rdtsc() for high resolution
wall clock counter is through armv8 PMU subsystem.
The PMU cycle counter runs at CPU frequency, However,
access to PMU cycle counter from user space is not enabled
by default in the arm64 linux kernel.
It is possible to enable cycle counter at user space access
by configuring the PMU from the privileged mode (kernel space).

by default rte_rdtsc() implementation uses portable
cntvct_el0 scheme. Application can choose the PMU based
implementation with CONFIG_RTE_ARM_EAL_RDTSC_USE_PMU

Signed-off-by: Jerin Jacob <jerin.jacob@caviumnetworks.com>
Acked-by: Hemant Agrawal <hemant.agrawal@nxp.com>