HACK: arm64: dts: qcom: xiaomi-beryllium: enable serial debug

Marked as hack because it helps workaround the random display failure during some reboots. Issue is discussed here: https://gitlab.freedesktop.org/drm/msm/-/issues/46 and here: postmarketOS/pmaports#2518 (closed)

But, let's keep this even if the bug is fixed, because I got serial debugging work reliably in my Poco F1 atlast!!