powerpc/microwatt: Add SMP support

This adds support for Microwatt systems with more than one core, and
updates the device tree for a 2-core version.

The secondary CPUs are started and sent to spin in __secondary_hold
very early on, in the platform probe function.  The reason for doing
this is so that they are there when smp_release_cpus() gets called,
which is before the platform init_smp function or even the platform
setup_arch function gets called.

Note that having two CPUs in the device tree doesn't preclude
operation with only one CPU.  The SYSCON_CPU_CTRL register has a
read-only field which indicates the number of CPU cores, so
microwatt_init_smp() will only start as many CPU cores as are present
in the system, and any extra CPU device-tree nodes will just be
ignored.

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/Z5xt8aooKyXZv6Kf@thinks.paulus.ozlabs.org