arm64: dts: rockchip: correct ep-gpios for rk3399-sapphire

The endpoint control gpio for rk3399-sapphire boards is gpio2_a4,
so correct it now.

Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>