dev-arm: Add HDLcd DTB autogeneration

A Display has been defined. Its sole purpose is to generate the device
tree node to be referenced by the HDLcd device. The encoder parameters
are based on the existing node defined in:

system/arm/dt/armv8.dts

Change-Id: I6cdeb0437dce207dbd0f2c65c16b224245eb74e1
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/20330
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>