powerpc/powernv: Move opal-api.h closer to the Skiboot version

This commit gets opal-api.h to mostly match the version in Skiboot as of
commit ea7d806ab0ba.

The exceptions are things which are not (currently) used in Linux.

Most of this is just whitespace and a few things moving around. I think
the diff is readable.

Also OpalMessageType became opal_msg_type, requiring a change in the
Linux code.

Finally Skiboot and Linux disagree on CAPI vs CXL, because CAPI means
something else in Linux. To handle that we just point the Linux wrapper,
which is named "cxl" to the OPAL token OPAL_PCI_SET_PHB_CAPI_MODE.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Reviewed-by: Stewart Smith <stewart@linux.vnet.ibm.com>