ethtool: Add 20G bit definitions

Add 20G supported and advertising bit definitions.
20G will be supported with the 57840 chips.

Signed-off-by: Yaniv Rosner <yanivr@broadcom.com>
Signed-off-by: Eilon Greenstein <eilong@broadcom.com>
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 include/linux/ethtool.h |    4 ++++
 1 files changed, 4 insertions(+), 0 deletions(-)
Signed-off-by: David S. Miller <davem@davemloft.net>