spacemit: k1: Add DDR firmware support to SPL

spacemit: k1: Add DDR firmware support to SPL

Include DDR initialization firmware in the SPL image. The firmware
path can be specified via the DDR_FW_FILE environment variable. If
the firmware is not found, an empty placeholder file is created to
allow the build to proceed without DDR initialization support.

Signed-off-by: Raymond Mao <raymond.mao@riscstar.com>
Signed-off-by: Guodong Xu <guodong@riscstar.com>
Tested-by: Songsong Zhang <sszhang@vsit.ai>