[PPC64] Enable opal console use as a GDB DBGPORT

This change makes it possible to use OPAL console as a GDB debug port.

Similar to uart and uart_phyp debug ports, it has to be enabled by
setting the hw.uart.dbgport variable to the serial console node
of the device tree.

Reviewed by: jhibbits
Differential Revision: https://reviews.freebsd.org/D22649