phy: rockchip-dp: should be a child device of the GRF

The displayport-phy is fully enclosed in the general register files (GRF).
Therefore as seen from the device-tree it shouldn't be a separate platform-
device but instead a sub-device of the GRF - using the simply-mfd mechanism.

The driver entered the kernel in the current merge-window, so we can still
adapt the binding without needing a fallback, as the binding hasn't been
released with a full kernel yet.

While the edp phy is fully part of the GRF, it doesn't have any separate
register set there, so doesn't get any register-area assigned.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Yakir Yang <ykk@rock-chips.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>