arm64: dts: rockchip: enable internal SPI flash for RockPro64.

The rockpro64 contains a nor-flash chip connected to spi1.

Signed-off-by: Andrius Štikonas <andrius@stikonas.eu>
[a number of cleanups]
Signed-off-by: Heiko Stuebner <heiko@sntech.de>