powerpc: Use 64k io pages when we never see an HEA

When we never get around to seeing an HEA ethernet adapter, there's
no point in restricting ourselves to 4k IO page size.

This speeds up IO maps when CONFIG_IBMEBUS is disabled.

[ Updated the test to also lift the restriction on arch 2.07
  (Power 8) which cannot have an HEA
 -- BenH ]

Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
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