arm64: dts: qcom: sdm845-xiaomi-beryllium: Add reserved memory region

Put cont splash memory region under the reserved-memory as
confirmed by the downstream code for both Tianma and EBBG variants.

Signed-off-by: Amit Pundir <amit.pundir@linaro.org>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20230127063355.1775246-1-amit.pundir@linaro.org