arm64: dts: rockchip: fix &pinctrl phy sub nodename for rk3399-nanopi4

arm64: dts: rockchip: fix &pinctrl phy sub nodename for rk3399-nanopi4

A test with the command below gives for example this error:

arch/arm64/boot/dts/rockchip/rk3399-nanopc-t4.dt.yaml: phy:
'#phy-cells' is a required property
arch/arm64/boot/dts/rockchip/rk3399-nanopi-m4.dt.yaml: phy:
'#phy-cells' is a required property
arch/arm64/boot/dts/rockchip/rk3399-nanopi-neo4.dt.yaml: phy:
'#phy-cells' is a required property

'phy' is a reserved nodename and should not be used for pinctrl,
so change it to 'gmac'.

make ARCH=arm64 dtbs_check
DT_SCHEMA_FILES=~/.local/lib/python3.5/site-packages/dtschema/schemas/
phy/phy-provider.yaml

Signed-off-by: Johan Jonker <jbx6244@gmail.com>
Link: https://lore.kernel.org/r/20200321215423.12176-4-jbx6244@gmail.com
Signed-off-by: Heiko Stuebner <heiko@sntech.de>