arm: mm: Don't free prohibited memmap entries

The VM subsystem assumes that there are valid memmap entries to
the bank end aligned to MAX_ORDER_NR_PAGES. It will try and read
these page structs, and so we cannot free any memmap entries that
it may inspect.

Signed-off-by: Michael Bohan <mbohan@codeaurora.org>
Signed-off-by: Daniel Walker <dwalker@codeaurora.org>