[PPC64] Enable phyp vty use as a GDB DBGPORT

This change makes it possible to use a POWER Hypervisor virtual
terminal device (phyp vty) as a GDB debug port.

Similar to the uart debug port, it has to be enabled by setting
the hw.uart_phyp.dbgport variable to the vty node of the device
tree.

Reviewed by: jhibbits
Differential Revision: https://reviews.freebsd.org/D22205