net: stmmac: Implement RX Coalesce Frames setting

Add support for coalescing RX path by specifying number of frames which
don't need to have interrupt on completion bit set.

This is only available when RX Watchdog is enabled.

Acked-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Jose Abreu <joabreu@synopsys.com>
Signed-off-by: David S. Miller <davem@davemloft.net>