xen/arm: Add p2m_set_permission and p2m_shatter_page helpers.

Signed-off-by: Tamas K Lengyel <tklengyel@sec.in.tum.de>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Reviewed-by: Julien Grall <julien.grall@linaro.org>
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v8: Determine level_shift in p2m_shatter_page instead of passing as argument.