cxl: read PHB indications from the device tree

cxl: read PHB indications from the device tree

BugLink: http://bugs.launchpad.net/bugs/1762448
Configure the P9 XSL_DSNCTL register with PHB indications found
in the device tree, or else use legacy hard-coded values.

Signed-off-by: Philippe Bergheaud <felix@linux.vnet.ibm.com>
Reviewed-by: Frederic Barrat <fbarrat@linux.vnet.ibm.com>
Reviewed-by: Christophe Lombard <clombard@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
(cherry picked from commit 9dbcbfa1fe0c3b556e889ea213a73eb80d74307b)
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>