cxl: Remove driver

Remove the cxl driver that provides support for the IBM Coherent
Accelerator Processor Interface. Revert or clean up associated code in
arch/powerpc that is no longer necessary.

cxl has received minimal maintenance for several years, and is not
supported on the Power10 processor. We aren't aware of any users who are
likely to be using recent kernels.

Thanks to Mikey Neuling, Ian Munsie, Daniel Axtens, Frederic Barrat,
Christophe Lombard, Philippe Bergheaud, Vaibhav Jain and Alastair
D'Silva for their work on this driver over the years.

Signed-off-by: Andrew Donnellan <ajd@linux.ibm.com>
Acked-by: Frederic Barrat <fbarrat@linux.ibm.com>
Acked-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://patch.msgid.link/20250219070007.177725-2-ajd@linux.ibm.com