mlx5: add support for page_pool_get_stats

This change adds support for the page_pool_get_stats API to mlx5. If the
user has enabled CONFIG_PAGE_POOL_STATS in their kernel, ethtool will
output page pool stats.

Signed-off-by: Joe Damato <jdamato@fastly.com>
Acked-by: Saeed Mahameed <saeed@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>