libcxgb: add library module for Chelsio drivers

Add common library module(libcxgb.ko) for
Chelsio drivers to remove duplicate code.

Code for iSCSI DDP Page Pod Manager is moved
from cxgb4.ko to libcxgb.ko. Earlier only cxgbit.ko
was using this code, now cxgb3i and cxgb4i will
also use common Page Pod manager code.

In future this module will have common connection
management and hardware specific code that can be
shared by multiple Chelsio drivers.

Signed-off-by: Varun Prakash <varun@chelsio.com>
Reviewed-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: David S. Miller <davem@davemloft.net>