arm: drop intermediate cpu_model -> cpu type parsing and use cpu type directly

there are 2 use cases to deal with:
  1: fixed CPU models per board/soc
  2: boards with user configurable cpu_model and fallback to
     default cpu_model if user hasn't specified one explicitly

For the 1st
  drop intermediate cpu_model parsing and use const cpu type
  directly, which replaces:
     typename = object_class_get_name(
           cpu_class_by_name(TYPE_ARM_CPU, cpu_model))
     object_new(typename)
  with
     object_new(FOO_CPU_TYPE_NAME)
  or
     cpu_generic_init(BASE_CPU_TYPE, "my cpu model")
  with
     cpu_create(FOO_CPU_TYPE_NAME)

as result 1st use case doesn't have to invoke not necessary
translation and not needed code is removed.

For the 2nd
 1: set default cpu type with MachineClass::default_cpu_type and
 2: use generic cpu_model parsing that done before machine_init()
    is run and:
    2.1: drop custom cpu_model parsing where pattern is:
       typename = object_class_get_name(
           cpu_class_by_name(TYPE_ARM_CPU, cpu_model))
       [parse_features(typename, cpu_model, &err) ]

    2.2: or replace cpu_generic_init() which does what
         2.1 does + create_cpu(typename) with just
         create_cpu(machine->cpu_type)
as result cpu_name -> cpu_type translation is done using
generic machine code one including parsing optional features
if supported/present (removes a bunch of duplicated cpu_model
parsing code) and default cpu type is defined in an uniform way
within machine_class_init callbacks instead of adhoc places
in boadr's machine_init code.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Message-Id: <1505318697-77161-6-git-send-email-imammedo@redhat.com>
Reviewed-by: Alistair Francis <alistair.francis@xilinx.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>