arm/psci: Test that CPU 1 has been successfully brought online

For the PSCI CPU_ON function test, all other CPUs perform a CPU_ON call
that target CPU 1. The test is considered a success if CPU_ON returns PSCI
SUCCESS exactly once, and for the rest of the calls PSCI ALREADY_ON.

Enhance the test by checking that CPU 1 is actually online and able to
execute code. Also make the test more robust by checking that the CPU_ON
call returns, instead of assuming that it will always succeed and
hanging indefinitely if it doesn't.

Since the CPU 1 thread is now being set up properly by kvm-unit-tests
when being brought online, it becomes possible to add other tests in the
future that require all CPUs.

The include header order in arm/psci.c has been changed to be in
alphabetic order. This means moving the errata.h include before
libcflat.h, which causes compilation to fail because of missing includes
in errata.h. Fix that also by including the needed header in errata.h.

Reviewed-by: Andrew Jones <andrew.jones@linux.dev>
Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>
Signed-off-by: Andrew Jones <andrew.jones@linux.dev>