Documentation: Document PowerPC kernel DEXCR interface

Describe the DEXCR and document how to configure it.

Signed-off-by: Benjamin Gray <bgray@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230616034846.311705-9-bgray@linux.ibm.com