TI DaVinci EMAC : Implement interrupt pacing functionality.

DaVinci EMAC module includes an interrupt pacing block that can
be programmed to throttle the rate at which interrupts are
generated. This patch implements interrupt pacing logic that can
be controlled through the ethtool interface(only rx_coalesce_usecs
param is honored)

Signed-off-by: Sriramakrishnan <srk@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>