tools: imx8image: add upower image support

Part of the upower management was included in a previous commit [1].
This patch only adds the bits required to properly parse a config file
that would include the binary as follows:
IMAGE PWR upower.bin

[1] 6ec65c85 (tools: image: support i.MX93)

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