net: fman: Export/rename some common functions

In preparation for moving each of the initialization functions to their
own file, export some common functions so they can be re-used. This adds
an fman prefix to set_multi to make it a bit less genericly-named.

Signed-off-by: Sean Anderson <sean.anderson@seco.com>
Acked-by: Camelia Groza <camelia.groza@nxp.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>