rockchip: avoid out-of-bounds when computing cpuid

rockchip: avoid out-of-bounds when computing cpuid

The expected length of the cpuid, as passed with cpuid_length,
determines the size of cpuid_str string. Therefore, care should be taken
to make sure nothing is accessing data out-of-bounds.

Instead of using hardcoded values, derive them from cpuid_length.

Cc: Quentin Schulz <foss+uboot@0leil.net>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
Reviewed-by: Dragan Simic <dsimic@manjaro.org>
Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>