cxl: Fix build when CONFIG_DEBUG_FS=n

Stub out the debugfs functions so that the build doesn't break when
CONFIG_DEBUG_FS=n.

Reported-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Andrew Donnellan <andrew.donnellan@au1.ibm.com>
Acked-by: Ian Munsie <imunsie@au1.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>