cpu: imx: fix the CPU type field width

Increase one more bit to cover all CPU types. Otherwise it shows
wrong CPU info on some platforms, such as i.MX8M Plus:

    U-Boot 2024.04+g674440bc73e+p0 (Jun 06 2024 - 10:05:34 +0000)

    CPU:   NXP i.MX8MM Rev1.1 A53 at 4154504685 MHz at 30C

    Model: NXP i.MX8MPlus LPDDR4 EVK board

Signed-off-by: Hou Zhiqiang <Zhiqiang.Hou@nxp.com>