[POWERPC] Allow duplicate lmb_reserve() calls

At present calling lmb_reserve() (and hence lmb_add_region()) twice
for exactly the same memory region will cause strange behaviour.

This makes life difficult when booting from a flat device tree with
memory reserve map.  Which regions are automatically reserved by the
kernel has changed over time, so it's quite possible a newer kernel
could attempt to auto-reserve a region which is also explicitly listed
in the device tree's reserve map, leading to trouble.

This patch avoids the problem by making lmb_reserve() ignore a call to
reserve a previously reserved region.  It also removes a now redundant
test designed to avoid one specific case of the problem noted above.

At present, this patch deals only with duplicate reservations of an
identical region.  Attempting to reserve two different, but
overlapping regions will still cause problems.  I might post another
patch later dealing with this case, but I'm avoiding it now since it
is substantially more complicated to deal with, less likely to occur
and more likely to indicate a genuine bug elsewhere if it does occur.

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

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