rockchip: spi: enable support for the rk_spi driver for the RK3399

rockchip: spi: enable support for the rk_spi driver for the RK3399

The existing Rockchip SPI (rk_spi.c) driver also matches the hardware
block found in the RK3399.  This has been confirmed both with SPI NOR
flashes and general SPI transfers on the RK3399-Q7 for SPI1 and SPI5.

This change adds the 'rockchip,rk3399-spi' string to its compatible
list to allow reuse of the existing driver.

X-AffectedPlatforms: RK3399-Q7
Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Tested-by: Jakob Unterwurzacher <jakob.unterwurzacher@theobroma-systems.com>
Acked-by: Simon Glass <sjg@chromium.org>