stm32mp: cmd_stm32key: use sub command

Simplify parsing the command argument by using
the macro U_BOOT_CMD_WITH_SUBCMDS.

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>
1 file changed