Implement configurable descriptor size in ftgmac100

Implement configurable descriptor size in ftgmac100

The hardware supports configurable descriptor sizes, configured in the DBLAC
register.

Most drivers use the default 4 word descriptor, which is currently hardcoded,
but Aspeed SDK configures 8 words to store extra data.

Signed-off-by: Erik Smit <erik.lucas.smit@gmail.com>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
[PMM: removed unnecessary parens]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>