Add a manual page for axp(4) / AMD 10G Ethernet driver

Submitted by: Rajesh Kumar <rajesh1 dot kumar at amd dot com>
Reviewed by: bcr, brueffer, rpokala
Differential Revision: https://reviews.freebsd.org/D27800

(cherry picked from commit 1f73236b0302cdf47630c99687c2dfd4e514ab54)