target-arm: Use correct memory attributes for page table walks

target-arm: Use correct memory attributes for page table walks

Factor out the page table walk memory accesses into their own function,
so that we can specify the correct S/NS memory attributes for them.
This will also provide a place to use the correct endianness and
handle the need for a stage-2 translation when virtualization is
supported.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>