plat/xen: Fix implicit int cast build error

plat/xen: Fix implicit int cast build error

Building xen on arm64 with Clang would fail due to an implicit
cast from char * to __uptr.

Signed-off-by: Andrei Stan <andreistan2003@gmail.com>
Reviewed-by: Oleksii Moisieiev <oleksii_moisieiev@epam.com>
Reviewed-by: Stefan Jumarea <stefanjumarea02@gmail.com>
Reviewed-by: Razvan Deaconescu <razvand@unikraft.io>
Reviewed-by: Simon Kuenzer <simon@unikraft.io>
Approved-by: Simon Kuenzer <simon@unikraft.io>
GitHub-Closes: #1518