sunxi: Fix arm64 fdtfile variable

Currently $fdtfile is constructed from CONFIG_DEFAULT_TREE, containing
the filename. However on arm64 that file is located in an allwinner
subdirectory.

To avoid the need for users/distros symlinking the .dtb files, prepend
the vendor directory for ARM64.

This aligns Pine64 with other boards such as Raspberry Pi 3.

Signed-off-by: Andreas Färber <afaerber@suse.de>
Reviewed-by: Alexander Graf <agraf@suse.de>
Reviewed-by: Jagan Teki <jagan@openedev.com>