powerpc/powernv: Move OPAL API definitions to opal-api.h

We'd like to get to the stage where the OPAL API is defined in a header
that is identical between Linux and Skiboot.

As step one, split the bits that actually define the API into
opal-api.h. The Linux specific parts stay in opal.h.
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Acked-by: Stewart Smith <stewart@linux.vnet.ibm.com>