dt: add /cpus/ibm, powerpc-cpu-features device tree bindings

This is a new CPU feature advertising interface that is fine-grained,
extensible, aware of privilege levels, and gives control of features
to all levels of the stack (firmware, hypervisor, and OS).

The design and binding specification is described in detail in doc/.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
[stewart: fix maybe-uninitialized warning from older GCC, doc cleanup]
Signed-off-by: Stewart Smith <stewart@linux.vnet.ibm.com>

core/Makefile.inc

@@ -9,7 +9,7 @@ CORE_OBJS += vpd.o hostservices.o platform.o nvram.o nvram-format.o hmi.o
 CORE_OBJS += console-log.o ipmi.o time-utils.o pel.o pool.o errorlog.o
 CORE_OBJS += timer.o i2c.o rtc.o flash.o sensor.o ipmi-opal.o
 CORE_OBJS += flash-subpartition.o bitmap.o buddy.o pci-quirk.o powercap.o psr.o
-CORE_OBJS += pci-dt-slot.o direct-controls.o
+CORE_OBJS += pci-dt-slot.o direct-controls.o cpufeatures.o
 
 ifeq ($(SKIBOOT_GCOV),1)
 CORE_OBJS += gcov-profiling.o