mx7: Fix the get_cpu_speed_grade_hz() return values

According to the MX7D fuse map the following speed grades are available:

800  MHz
500  MHz
1000 MHz
1200 MHz

So simply return the real frequency that corresponds to the speed grade.

With this change we see on boot:

CPU:   Freescale i.MX7D rev1.2 1000 MHz (running at 792 MHz)

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>