cpufreq: add virtual-cpufreq driver

Introduce a virtualized cpufreq driver for guest kernels to improve
performance and power of workloads within VMs.

This driver does two main things:

1. Sends the frequency of vCPUs as a hint to the host. The host uses the
hint to schedule the vCPU threads and decide physical CPU frequency.

2. If a VM does not support a virtualized FIE(like AMUs), it queries the
host CPU frequency by reading a MMIO region of a virtual cpufreq device
to update the guest's frequency scaling factor periodically. This enables
accurate Per-Entity Load Tracking for tasks running in the guest.

Co-developed-by: Saravana Kannan <saravanak@google.com>
Signed-off-by: Saravana Kannan <saravanak@google.com>
Signed-off-by: David Dai <davidai@google.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>