net/cnxk: support pre L2 switch header type

net/cnxk: support pre L2 switch header type

Adding changes to configure switch header type pre_l2 for cnxk.
pre_l2 headers are custom headers placed before the ethernet
header. Along with switch header type, user needs to provide the
offset within the custom header that holds the size of the
custom header and mask for the size within the size offset.

Signed-off-by: Kiran Kumar K <kirankumark@marvell.com>
Reviewed-by: Satheesh Paul <psatheesh@marvell.com>