xtensa: fix sparse warning when converting to uncache pointer

The argument to arch_xtensa_uncached_ptr() is marked as in
the cached address space. However, when it is being passed to
z_xtropflip(), it is simply casted to uint32_t, thus resulting
in sparse warning about discarding "__cached" address space.
This adds __sparse_force to the cast to suppress the warning.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>