lib: shell: replace strtol with strtoul in cmd_load for address parsing

Addresses in cmd_load() should always be unsigned. Previously, strtol()
was used, which is limited to signed long values, causing issues with
addresses >= 0x80000000. This commit replaces strtol() with strtoul(),
ensuring proper handling of the full 32-bit address space.

Fixes #81343

Signed-off-by: Aaron Fontaine <aaron.fontaine@dojofive.com>
Signed-off-by: Jakub Rzeszutko <jakub.rzeszutko@verkada.com>