cxl: Add support for CAPP DMA mode

cxl: Add support for CAPP DMA mode

This adds support for using CAPP DMA mode, which is required for XSL
based cards such as the Mellanox CX4 to function.

This is currently an RFC as it depends on the corresponding support to
be merged into skiboot first, which was submitted here:
http://patchwork.ozlabs.org/patch/625582/

In the event that the skiboot on the system does not have the above
support, it will indicate as such in the kernel log and abort the init
process.

Signed-off-by: Ian Munsie <imunsie@au1.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>