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 <[e-mail]>
Acked-by: Shuah Khan <[e-mail]>
Signed-off-by: David Matlack <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alex Williamson <[e-mail]>

drivers/dma/idxd/registers.h

···
 #ifndef _IDXD_REGISTERS_H_
 #define _IDXD_REGISTERS_H_
 
+#ifdef __KERNEL__
 #include <uapi/linux/idxd.h>
+#else
+#include <linux/idxd.h>
+#endif
 
 /* PCI Config */
 #define PCI_DEVICE_ID_INTEL_DSA_GNRD	0x11fb