ARM: dts: rockchip: Add spi_flash label to rk3288-veyron

ARM: dts: rockchip: Add spi_flash label to rk3288-veyron

The u-boot,spl-boot-order property requires a label at a boot
device node. In order to migrate to OF_UPSTREAM more easier
add a spi_flash label to the rk3288-veyron.dtsi file.

Signed-off-by: Johan Jonker <jbx6244@gmail.com>
Link: https://patch.msgid.link/8a423a6f-bfdc-4947-aef9-35ee7c4f6ca2@gmail.com
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
[ upstream commit: ac7116a626e00d3024adedb43b74bdf2ce45efc2 ]

(cherry picked from commit 38e6f7cd300f66cc9f657cd0e8881a01acc664d0)
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>