ARM: dts: rmobile: Add PHY reset GPIO

This patch got dropped during the update of DTs to 4.14, re-add it
back. This adds reset GPIOs to the ethernet PHYs.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>
Signed-off-by: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>