[POWERPC] Small clarification of initrd handling

This patch makes the handling of the initrd (or initramfs) in the
zImage wrapper a little easier to follow.  Instead of passing the
initrd addresses out from prep_kernel() via the cryptic a1 and a2
parameters, use the global struct add_range, 'initrd'.  prep_kernel()
already passes information through the 'vmlinux' addr_range struct, so
this seems like a reasonable extension.

Some comments also clarify the logic with prep_kernel(): we use an
initrd included in the zImage if present, otherwise we use an initrd
passed in by the bootloader in the a1 and a2 parameters (yaboot, at
least, uses this mechanism to pass an initrd).

Signed-off-by: David Gibson <dwg@au1.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>