rockchip: spi: enable support for the rk_spi driver for the RK3368

For the RK3368, we can reuse the SPI driver (although we'll have to
eventually investigate whether it can be merged with the
designware_spi.c driver) also used for the RK3288 and RK3399.
This adds the necessary compatible string to support the RK3368.

Note that the assumption that GPLL will be clocked at 594MHz is not
true for the RK3368, but this will not lead to incorrect functioning
(just to a lower-than-expected SPI operating frequency): this has been
documented in the driver, so it doesn't cause any headaches when
someone next needs to touch the clock code of this driver.

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>