arm64: dts: sdm845: Add zap shader region for GPU

Some Adreno GPU targets require a special zap shader to bring the GPU
out of secure mode. Define a region to allocate and store the zap
shader.

Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>
[bjorn: Rebase ontop of recent reserved-memory patch]
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>

Signed-off-by: Andy Gross <agross@kernel.org>