Add PSCI monitor to sysutils/rpi-firmware

Build PSCI monitor and add it to common firmware files for RaspberrPi
platform. This is pre-requisite for migration of sysutils/u-boot-rpi3 to
u-boot-master framework.

Submitted by: gonzo
Reviewed by: bapt
Approved by: bapt
Differential Revision: https://reviews.freebsd.org/D14629