net/macb: Adding comments to various #defs to make interpretation easier

This change is to help improve at-a-glace knowledge of the purpose of the
various Cadence MACB/GEM registers. Comments are more helpful for human
readability than short acronyms.

Describe various #define varibles Cadence MACB/GEM registers as documented
in Xilinix's "Zynq-7000 All Programmable SoC TechnicalReference Manual, v1.9.1
(UG-585)"

Signed-off-by: Xander Huff <xander.huff@ni.com>
Signed-off-by: David S. Miller <davem@davemloft.net>