nfp: abm: add helpers for configuring queue marking levels

Queue levels for simple ECN marking are stored in _abi_nfd_out_q_lvls_X
symbol, where X is the PCIe PF id.  Find out the location of that symbol
and add helpers for modifying it.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>