[PPC64] Fix ofw_initrd

Before this change, OFW initrd (as md) handling code was simulating an ofwbus
device. But as there isn't really a Device Tree (DT) node representing OFW
initrd (it is specified in 2 properties under /chosen), its driver was in fact
stealing other driver's DT node. This was noticed after MD_ROOT_MEM became
default and QEMU's USB keyboard stopped working under VNC.

This change consists in simplifying the process of detection and mapping of
initrd memory, turning it into a simple startup step, instead of trying to
simulate a device.

Reviewed by: jhibbits
Differential Revision: https://reviews.freebsd.org/D20553