MLK-20798 imx8: spl: Fix container header parser issue

MLK-20798 imx8: spl: Fix container header parser issue

Current container parser only load 0x400 as container header size.
However, the signature block in container header may exceed 0x400 size,
when using certificate or 4096bits RSA keys to sign image, so we
have to load the entire header according to container length field.
Otherwise the container authentication will fail

Signed-off-by: Ye Li <ye.li@nxp.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
(cherry picked from commit f435435f2e8367dc3f689d6ba946441a54acad0a)