common: Fix cpu nr type which is always unsigned type

common: Fix cpu nr type which is always unsigned type

cpu_cmd() is reading cpu number via simple_strtoul() which is always
unsigned type.
Platform code implementations are not expecting that nr can be negative
and there is not checking in the code for that too.

This patch is using u32 type for cpu number to make sure that platform
code get proper value range.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>