arm64: dts: qcom: sdm845-polaris: add missing touchscreen child node reg

Add missing reg property to touchscreen child node to fix dtbs W=1 warnings:

  Warning (unit_address_vs_reg): /soc@0/geniqup@ac0000/i2c@a98000/touchscreen@20/rmi4-f12@12: node has a unit name, but no reg or ranges property

Fixes: be497abe ("arm64: dts: qcom: Add support for Xiaomi Mi Mix2s")
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Reviewed-by: Molly Sophia <mollysophia379@gmail.com>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20230419211856.79332-18-krzysztof.kozlowski@linaro.org