dt-bindings: gpu: Update properties for bus scaling table

The bus scaling table is a child node with a new compatible
string property. This design helps to define multiple bus
scaling tables based on the type of ddr in the system.

Change-Id: I8b060dc8ce9aa0f541299f02dd1b397ef2f1b0bb
Signed-off-by: Harshdeep Dhatt <hdhatt@codeaurora.org>