Add firmware command interface to read and write PPTB and PBMC
registers.

PPTB register enables mappings priority to a specific receive buffer.

PBMC registers enables changing the receive buffer's configuration such
as buffer size, xon/xoff thresholds, buffer's lossy property and
buffer's shared property.

Signed-off-by: Huy Nguyen <huyn@mellanox.com>
Reviewed-by: Parav Pandit <parav@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>