u-boot: v2026.04: own the rk3588 blueberry & mixtile DTs (drop kernel symlinks)

These two u-boot device trees were symlinks into the kernel patch archive:

  dt_upstream_rockchip/rk3588-blueberry-edge-v12-linux.dts
      -> ../../../kernel/archive/rockchip64-7.0/dt/...
  dt_upstream_rockchip/rk3588-mixtile-blade3.dts
      -> ../../../kernel/archive/rockchip64-6.18/dt/...

u-boot carries its own device tree — it does not consume the kernel's. The
symlink-shared-DT change (7ef9f60) coupled the u-boot build to the kernel
patch tree, so when the edge kernel archive moved (rockchip64-7.0 dropped, edge
is now 7.1) the blueberry symlink dangled and the u-boot artifact build failed:

  make[2]: No rule to make target
    'dts/upstream/src/arm64/rockchip/rk3588-blueberry-edge-v12-linux.dtb'

Restore both as real, self-contained files in the u-boot patch set (materialised
from the current board DT), so the u-boot DTs no longer depend on the kernel
archive and can't be broken by kernel-branch churn. mixtile content is unchanged
(its 6.18 target still resolved); blueberry takes the current edge content.

Signed-off-by: Igor Pecovnik <igor@armbian.com>

Author: igorpecovnik