[ARM] initrd: claim initrd memory exclusively

Claim the initrd memory exclusively, and order other memory
reservations beforehand.  This allows us to determine whether
the initrd memory was overwritten, and disable the initrd in
that case.

This avoids a 'bad page state' bug.

Tested-by: Ralph Siemsen <ralphs@netwinder.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>