powerpc: Add cxl context to device archdata

Add cxl context pointer to archdata.  We'll want to create one of these for cxl
PCI devices.  Put them here until we can get a pci_dev specific private data.

This location was suggested by benh.

Signed-off-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>