arm64: dts: rockchip: Enable SPI NOR flash on Rock64

The Pine64 Rock64 board comes with a GigaDevice GD25Q128CSIG
or GD25Q127CSIG chip, which is a 128 Mbit SPI NOR flash chip
that supports the JEDEC read-ID command.

This patch enables the SPI controller and adds a device node
for the flash chip using the generic "jedec,spi-nor" comaptible.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>