app/eventdev: add options for mbuf and packet sizes

Add options to set mbuf size and max packet size which allow the user to
enable jumbo frames and Rx/Tx scatter gather.
Arrange `struct evt_options` based on ascending order of data type to
make it more readable.

Packet mbuf size can be modified by using `--mbuf_sz=N`.
Max packet size can be modified by using `--max_pkt_sz=N`.
These options are only applicable `pipeline_atq` and `pipeline_queue`
tests.

Signed-off-by: Pavan Nikhilesh <pbhagavatula@marvell.com>
Acked-by: Jerin Jacob <jerinj@marvell.com>