optee: move optee_verify_header() to common ...

Subsequent patches will use this to verify the header in the PBL, move
it to common to make it potentially available for both.

Signed-off-by: Rouven Czerwinski <r.czerwinski@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>