ARM: dts: rockchip: Assign RK3288 EDP_24M input centrally

The EDP 24M clock can be fed either by an SoC internal fixed clock or
from an external IC. Change the default parent to the internal clock in
the main rk3288 dtsi, to ensure (by default) it gets setup with a
non-orphaned clock (hardware defaults to the externa clock).

This prevents potential issues when the clock framework get support for
deferring on orphaned clocks, while specific boards can always change
the parent clock if an external input is preferred.

Signed-off-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>