dmaengine: idxd: Allow registers.h to be included from tools/

dmaengine: idxd: Allow registers.h to be included from tools/

Allow drivers/dma/idxd/registers.h to be included from userspace in
tools/ by adjusting the include path to uapi/linux/idxd.h if __KERNEL__
is not defined.

A subsequent commit will use registers.h to implement a userspace driver
for Intel DSA devices in tools/testing/selftests/vfio.

Acked-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>
Acked-by: Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: David Matlack <dmatlack@google.com>
Link: https://lore.kernel.org/r/20250822212518.4156428-22-dmatlack@google.com
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>