acpi: cpuhp: spec: clarify 'CPU selector' register usage and endianness

* Move reserved registers to the top of the section, so reader would be
  aware of effects when reading registers description.
* State registers endianness explicitly at the beginning of the section
* Describe registers behavior in case of 'CPU selector' register contains
  value that doesn't point to a possible CPU.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <1575896942-331151-5-git-send-email-imammedo@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

docs/specs/acpi_cpu_hotplug.txt

@@ -30,6 +30,18 @@ Register block base address:
 Register block size:
     ACPI_CPU_HOTPLUG_REG_LEN = 12
 
+All accesses to registers described below, imply little-endian byte order.
+
+Reserved resisters behavior:
+   - write accesses are ignored
+   - read accesses return all bits set to 0.
+
+The last stored value in 'CPU selector' must refer to a possible CPU, otherwise
+  - reads from any register return 0
+  - writes to any other register are ignored until valid value is stored into it
+On QEMU start, 'CPU selector' is initialized to a valid value, on reset it
+keeps the current value.
+
 read access:
     offset:
     [0x0-0x3] reserved
@@ -86,9 +98,3 @@ write access:
                  ACPI_DEVICE_OST QMP event from QEMU to external applications
                  with current values of OST event and status registers.
             other values: reserved
-
-Selecting CPU device beyond possible range has no effect on platform:
-   - write accesses to CPU hot-plug registers not documented above are
-     ignored
-   - read accesses to CPU hot-plug registers not documented above return
-     all bits set to 0.