On the Hummingbird A31 board, the RTL8211E ethernet phy has its reset
line connect to a gpio pin, instead of floating like on other boards.
Add the stmmac properties for describing the reset gpio.

The reset delays were taken from the RTL8211E datasheet.
Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>