dt-bindings: ata: dwc-ahci: add Rockchip RK3588

This adds Rockchip RK3588 AHCI binding. In order to narrow down the
allowed clocks without bloating the generic binding, the description
of Rockchip's AHCI controllers has been moved to its own file.

Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Reviewed-by: Serge Semin <fancer.lancer@gmail.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Damien Le Moal <dlemoal@kernel.org>