smp: Allow booting a specific subset of CPUs

In a heterogenous multiprocessor system, specifying the
'maxcpus' parameter on the kernel command line does not
provide sufficient control over which CPUs are brought
online at kernel boot time, since CPUs may have nonuniform
performance characteristics. Thus, we introduce a
'boot_cpus' command line argument, allowing the user to
explicitly specify the list of CPUs that shall be brought
online during kernel boot.

Change-Id: I5f119e23202660941fa7be8c4e6dd91a82365451
Signed-off-by: Stepan Moskovchenko <stepanm@codeaurora.org>