docs: powerpc: cxl.rst: mark two section titles as such

The User API chapter contains two sub-chapters. Mark them as
such.

Signed-off-by: Mauro Carvalho Chehab <[e-mail]>
Acked-by: Andrew Donnellan <[e-mail]>
Link: https://lore.kernel.org/r/190d67[e-mail]
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/powerpc/cxl.rst

···
 ========
 
 1. AFU character devices
+^^^^^^^^^^^^^^^^^^^^^^^^
 
     For AFUs operating in AFU directed mode, two character device
     files will be created. /dev/cxl/afu0.0m will correspond to a
···
 
 
 2. Card character device (powerVM guest only)
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
     In a powerVM guest, an extra character device is created for the
     card. The device is only used to write (flash) a new image on the