ofw: Load initrd file

This is an OFW initrd module that would load the initrd from device tree
parameters and give the to the md driver.

With this patch, it is possible to pass a rootfs image through kexec in PowerNV
mode (powerpc64). In order to user it, you should set the MD_ROOT_MEM option in
your kernel configuration.

Reviewed by: jhibbits
Approved by: jhibbits (mentor)
Differential Revision: https://reviews.freebsd.org/D15705