KVM: arm64: Provide a PV_TIME device to user space

Allow user space to inform the KVM host where in the physical memory
map the paravirtualized time structures should be located.

A device is created which provides the base address of an array of
Stolen Time (ST) structures, one for each VCPU. There must be (64 *
total number of VCPUs) bytes of memory available at this location.

The address is given in terms of the physical address visible to
the guest and must be page aligned. The guest will discover the address
via a hypercall.

Signed-off-by: Steven Price <steven.price@arm.com>