MAINTAINERS: Update cxl maintainers

Fred has taken over the cxl maintenance I was doing.  This updates the
MAINTAINERS file to reflect this.

It also removes a duplicate entry in the files covered and adds an entry
for the CXL PCI code in arch/powerpc.

Signed-off-by: Michael Neuling <mikey@neuling.org>
Reviewed-by: Andrew Donnellan <andrew.donnellan@au1.ibm.com>
Reviewed-by: Andrew Donnellan <andrew.donnellan@au1.ibm.com>
Acked-by: Frederic Barrat <fbarrat@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>