kconfig,doc: Limit the number of AP CPUs to 4 cores

The number of available Primary Compute CPUs is currently limited
to 4 due to a reboot-related issue. After initiating a reboot from
Linux, the Out-of-Reset Platform Fault Detection Interface (PFDI)
fails to re-enable all CPU cores.

Signed-off-by: Ziad Elhanafy <ziad.elhanafy@arm.com>