stmmac: update the doc with new info about the driver's debug (v3)

 o enh_desc.c: functions for handling enhanced descriptors

 o norm_desc.c: functions for handling normal descriptors

5) TODO:

5) Debug Information

The driver exports many information i.e. internal statistics,

debug information, MAC and DMA registers etc.

These can be read in several ways depending on the

type of the information actually needed.

For example a user can be use the ethtool support

to get statistics: e.g. using: ethtool -S ethX

(that shows the Management counters (MMC) if supported)

or sees the MAC/DMA registers: e.g. using: ethtool -d ethX

Compiling the Kernel with CONFIG_DEBUG_FS and enabling the

STMMAC_DEBUG_FS option the driver will export the following

debugfs entries:

/sys/kernel/debug/stmmaceth/descriptors_status

  To show the DMA TX/RX descriptor rings

Developer can also use the "debug" module parameter to get

further debug information.

In the end, there are other macros (that cannot be enabled

via menuconfig) to turn-on the RX/TX DMA debugging,

specific MAC core debug printk etc. Others to enable the

debug in the TX and RX processes.

All these are only useful during the developing stage

and should never enabled inside the code for general usage.

In fact, these can generate an huge amount of debug messages.

6) TODO:

 o XGMAC is not supported.

 o Review the timer optimisation code to use an embedded device that will be

  available in new chip generations.