cxl: Workaround PE=0 hardware limitation in Mellanox CX4

The CX4 card cannot cope with a context with PE=0 due to a hardware
limitation, resulting in:

[   34.166577] command failed, status limits exceeded(0x8), syndrome 0x5a7939
[   34.166580] mlx5_core 0000:01:00.1: Failed allocating uar, aborting

Since the kernel API allocates a default context very early during
device init that will almost certainly get Process Element ID 0 there is
no easy way for us to extend the API to allow the Mellanox to inform us
of this limitation ahead of time.

Instead, work around the issue by extending the XSL structure to include
a minimum PE to allocate. Although the bug is not in the XSL, it is the
easiest place to work around this limitation given that the CX4 is
currently the only card that uses an XSL.

Signed-off-by: Ian Munsie <imunsie@au1.ibm.com>
Reviewed-by: Andrew Donnellan <andrew.donnellan@au1.ibm.com>
Reviewed-by: Frederic Barrat <fbarrat@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>