xen/arm/static-shmem: Static-shmem should be direct-mapped for direct-mapped domains

xen/arm/static-shmem: Static-shmem should be direct-mapped for direct-mapped domains

Currently, users are allowed to map static shared memory in a
non-direct-mapped way for direct-mapped domains. This can lead to
clashing of guest memory spaces. Also, the current extended region
finding logic only removes the host physical addresses of the
static shared memory areas for direct-mapped domains, which may be
inconsistent with the guest memory map if users map the static
shared memory in a non-direct-mapped way. This will lead to incorrect
extended region calculation results.

To make things easier, add restriction that static shared memory
should also be direct-mapped for direct-mapped domains. Check the
host physical address to be matched with guest physical address when
parsing the device tree. Document this restriction in the doc.

Signed-off-by: Henry Wang <xin.wang2@amd.com>
Signed-off-by: Stefano Stabellini <stefano.stabellini@amd.com>
Acked-by: Michal Orzel <michal.orzel@amd.com>