Add support for the XIVE XICS emulation mode for POWER9 systems

Summary:
POWER9 systems use a new interrupt controller, XIVE, managed through OPAL
firmware calls. The OPAL firmware includes support for emulating the previous
generation XICS presentation layer in addition to a new "XIVE Exploitation"
mode. As a stopgap until we have XIVE exploitation mode, enable XICS emulation
mode so that we at least have an interrupt controller.

Since the CPPR is local to the current CPU, it cannot be updated for APs when
initializing on the BSP. This adds a new function, directly called by the
powernv platform code, to initialize the CPPR on AP bringup.

Reviewed by: nwhitehorn
Differential Revision: https://reviews.freebsd.org/D15492