ptp: support ptp physical/virtual clocks conversion

Support ptp physical/virtual clocks conversion via sysfs.
There will be a new attribute n_vclocks under ptp physical
clock sysfs.

- In default, the value is 0 meaning only ptp physical clock
  is in use.
- Setting the value can create corresponding number of ptp
  virtual clocks to use. But current physical clock is guaranteed
  to stay free running.
- Setting the value back to 0 can delete virtual clocks and back
  use physical clock again.

Another new attribute max_vclocks control the maximum number of
ptp vclocks.

Signed-off-by: Yangbo Lu <yangbo.lu@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>