ptp: add VMware virtual PTP clock driver

Add a PTP clock driver called ptp_vmw, for guests running on VMware ESXi
hypervisor. The driver attaches to a VMware virtual device called
"precision clock" that provides a mechanism for querying host system time.
Similar to existing virtual PTP clock drivers (e.g. ptp_kvm), ptp_vmw
utilizes the kernel's PTP hardware clock API to implement a clock device
that can be used as a reference in Chrony for synchronizing guest time with
host.

The driver is only applicable to x86 guests running in VMware virtual
machines with precision clock virtual device present. It uses a VMware
specific hypercall mechanism to read time from the device.

Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Vivek Thampi <vithampi@vmware.com>
Signed-off-by: David S. Miller <davem@davemloft.net>