tools: imx8image: fix soc variable for ULP

Currently the ULP token sets the soc as IMX9, making it impossible to
differentiate the two families of processors.
However, since the 8ULP requires specific binaries like upower which do
not exist in 93, they need to be separated.

Fixes: 6ec65c85 (tools: image: support i.MX93)

Signed-off-by: Gary Bisson <bisson.gary@gmail.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>