powerpc/ps3: Add highmem repository read routines

Add repository helper routines to read highmem region info.

Bootloaders that preallocate highmem regions must place the
region info into the repository at these well known nodes.
These routines allow second stage kernles to read the region
info from those nodes.

Signed-off-by: Andre Heider <a.heider@gmail.com>
CC: Nathan Whitehorn <nwhitehorn@freebsd.org>
Signed-off-by: Geoff Levand <geoff@infradead.org>