xen/arm: Drop process_shm_chosen()

xen/arm: Drop process_shm_chosen()

There's no benefit in having process_shm_chosen() next to process_shm().
The former is just a helper to pass "/chosen" node to the latter for
hwdom case. Drop process_shm_chosen() and instead use process_shm()
passing NULL as node parameter, which will result in searching for and
using /chosen to find shm node (the DT full path search is done in
process_shm() to avoid expensive lookup if !CONFIG_STATIC_SHM). This
will simplify future handling of hw/control domain separation.

Signed-off-by: Michal Orzel <michal.orzel@amd.com>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>