perf: xgene: Add APM X-Gene SoC Performance Monitoring Unit driver

This patch adds a driver for the SoC-wide (AKA uncore) PMU hardware
found in APM X-Gene SoCs.

Signed-off-by: Tai Nguyen <ttnguyen@apm.com>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>