xen/arm: during efi boot, improve the check for usable memory

xen/arm: during efi boot, improve the check for usable memory

When booting via EFI, the EFI memory map has information about memory
regions and their type. Improve the check for the type and attribute of
each memory region to figure out whether it is usable memory or not.
This patch brings the check on par with Linux v5.5-rc6 and makes more
memory reusable as normal memory by Xen (except that Linux also reuses
EFI_PERSISTENT_MEMORY, which we do not).

Specifically, this patch also reuses memory marked as
EfiLoaderCode/Data, and it uses both Attribute and Type for the check
(Attribute needs to be EFI_MEMORY_WB).

Reported-by: Roman Shaposhnik <roman@zededa.com>
Signed-off-by: Stefano Stabellini <stefano.stabellini@xilinx.com>
Acked-by: Julien Grall <julien@xen.org>