octeontx2-af: Add macro to generate mbox handlers declarations

For every mailbox handler added to rvu, we are adding a function
declaration in rvu header file. Cleaned this up by adding a macro
to generate these declarations automatically.

Signed-off-by: Subbaraya Sundeep <sbhatta@marvell.com>
Signed-off-by: Sunil Goutham <sgoutham@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>