cxgbe(4): Add support for PPOD_EDRAM feature.

The driver queries the firmware to find out if it supports this feature
and enables it if it does. The firmware moves the iSCSI page pod region
to a lower address so that some of it is located in the faster on-chip
memory instead of external DDR.

Reviewed by: jhb@
MFC after: 3 weeks
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D34895