xen: Replace u64 with uint64_t in pdx_init_mask() and callers

xen: Replace u64 with uint64_t in pdx_init_mask() and callers

Xen is phasing out the use of u64 in favor of uint64_t. Therefore, the
instance of u64 in the pdx_init_mask() (and the callers) are now
replaced with uint64_t. Take the opportunity to also modify
srat_region_mask as this is used to store the result of pdx_init_mask().

Signed-off-by: Stefano Stabellini <stefanos@xilinx.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Acked-by: julien.grall@arm.com