cxl: Add userspace header file

This adds a header file for use by userspace programs wanting to interact with
the kernel cxl driver.  It defines structs and magic numbers required for
userspace to interact with devices in /dev/cxl/afuM.N.

Further documentation on this interface is added in a subsequent patch in
Documentation/powerpc/cxl.txt.

It also adds this new userspace header file to Kbuild so it's exported when
doing "make headers_installs".

Signed-off-by: Ian Munsie <imunsie@au1.ibm.com>
Signed-off-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>