sunxi: avoid double vendor prefix when CONFIG_OF_UPSTREAM is enabled

sunxi: avoid double vendor prefix when CONFIG_OF_UPSTREAM is enabled

When CONFIG_OF_UPSTREAM is enabled, the device tree name provided by SPL
already includes the vendor directory (e.g., "allwinner/board-name").

The existing logic in misc_init_r() unconditionally prepends "allwinner/"
for ARM64 builds, resulting in an incorrect path like
"allwinner/allwinner/board-name.dtb".

This patch modifies the logic to only prepend the vendor prefix if
CONFIG_OF_UPSTREAM is NOT enabled. This ensures compatibility with both
legacy builds and the new upstream devicetree structure.

Signed-off-by: Bohdan Chubuk <chbgdn@gmail.com>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>