I finally found the solution. The GPIO1 uses a different clock than the GPIO2-6 modules. You need to enable the functional and the interface clock in the CM_FCLKEN_WKUP and the CM_ICLKEN_WKUP registers
`
memory_mapped_io_t reg;
/* Enable Functional Clock of GPIO1 */
reg = (memory_mapped_io_t)CM_FCLKEN_WKUP;
(reg) = ((reg)) | (0x01 << 3);
/* Enable Interface Clock */
reg = (memory_mapped_io_t)CM_ICLKEN_WKUP;
(reg) = ((reg)) | (0x01 << 3);
`
After that the GPIO1 module is running and you can access it.