main/linux-postmarketos-qcom-sdm845: upgrade to 5.12.15

Upgrade the sdm845 linux kernel package to the latest 5.12 stable tag. This also includes the fix for the semi-random crashdump issue on the OnePlus 6 with 83e1c3850a1f1cc89ac344045fd81cb203d7340d ("arm64: dts: qcom: sdm845-oneplus-common: fix reserved-memory").

This should be backported to v21.06 so the fix can be included there too (Cc: @MartijnBraam ?).