net: mvneta: Allocate page for the descriptor

Instead of trying to allocate the exact amount of memory for each
descriptor use a page for each of them, it allows to simplify the
allocation management and increase the performance of the driver.

Based on the work of Yelena Krivosheev <yelena@marvell.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>