sfc: Add a function pointer to abstract write of host time into NIC shared memory

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>

diff --cc drivers/net/ethernet/sfc/net_driver.h
Simple merge

diff --cc drivers/net/ethernet/sfc/ptp.c
Simple merge

diff --cc drivers/net/ethernet/sfc/siena.c
Simple merge