imximage: Encase majority of header in __ASSEMBLY__ declaration

imximage: Encase majority of header in __ASSEMBLY__ declaration

Subsequent patches will want to include imageimage.h but in doing so
include it on an assembly compile path causing a range of compile errors.
Fix the errors pre-emptively by encasing the majority of the declarations
in imximage.h inside an ifdef __ASSEMBLY__ block.

Signed-off-by: Bryan O'Donoghue <[email protected]>
Cc: Utkarsh Gupta <[email protected]>
Cc: Breno Lima <[email protected]>
Cc: Fabio Estevam <[email protected]>
Signed-off-by: Bryan O'Donoghue <[email protected]>
Tested-by: Breno Lima <[email protected]>