dt-bindings: display: xlnx: Add VTC offset DT property

commit 424a1ea14d4187c879255298ac4742acf6bde343 from
https://github.com/Xilinx/linux-xlnx.git

 xlnx_rebase_v5.15

In DP Tx SS v3.0 the offset was at 0x1000 address and now it is at 0x8000
address. In order to avoid changes to driver if VTC offset changes again,
this value will be taken from device tree. Currently the expected offset
is 0x8000.

Signed-off-by: Rajesh Gugulothu <rajesh.gugulothu@amd.com>
Reviewed-by: Vishal Sagar <vishal.sagar@amd.com>
State: pending
Signed-off-by: Quanyang Wang <quanyang.wang@windriver.com>