dts: phy: add GPIO number and active state used for phy reset

The GEMGXL_RST line on HiFive Unleashed is pulled low and is
using GPIO number 12. Add these reset-gpio details to dt-node
using which the linux phylib can reset the phy.

Signed-off-by: Sagar Shrikant Kadam <sagar.kadam@sifive.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>