rockchip: dts: rk3399-firefly: update gmac parameter

Update the tx_delay and rx_delay to match the timing for
rk3399-firefly board to improve the stability of gmac data
transfer.

Signed-off-by: Kever Yang <kever.yang@rock-chips.com>
Acked-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>