arm64: dts: rockchip: add spi controller aliases on rk3399

There are 6 SPI controllers on RK3399 and they are all numbered in the
TRM, so let's add the appropriate aliases to the main DTSI so that any
RK3399-based board doesn't need to define the aliases themselves to
benefit from stable SPI indices in userspace.

Cc: Quentin Schulz <foss+kernel@0leil.net>
Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>
Link: https://lore.kernel.org/r/20240109-rk3399-spi-aliases-v1-1-2009e44e734a@theobroma-systems.com
Signed-off-by: Heiko Stuebner <heiko@sntech.de>