acpi: memory hotplug ACPI hardware implementation

- implements QEMU hardware part of memory hotplug protocol
  described at "docs/specs/acpi_mem_hotplug.txt"
- handles only memory add notification event for now

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

docs/specs/acpi_mem_hotplug.txt

@@ -0,0 +1,44 @@
+QEMU<->ACPI BIOS memory hotplug interface
+--------------------------------------
+
+ACPI BIOS GPE.3 handler is dedicated for notifying OS about memory hot-add
+events.
+
+Memory hot-plug interface (IO port 0xa00-0xa17, 1-4 byte access):
+---------------------------------------------------------------
+0xa00:
+  read access:
+      [0x0-0x3] Lo part of memory device phys address
+      [0x4-0x7] Hi part of memory device phys address
+      [0x8-0xb] Lo part of memory device size in bytes
+      [0xc-0xf] Hi part of memory device size in bytes
+      [0x10-0x13] Memory device proximity domain
+      [0x14] Memory device status fields
+          bits:
+              0: Device is enabled and may be used by guest
+              1: Device insert event, used to distinguish device for which
+                 no device check event to OSPM was issued.
+                 It's valid only when bit 1 is set.
+              2-7: reserved and should be ignored by OSPM
+      [0x15-0x17] reserved
+
+  write access:
+      [0x0-0x3] Memory device slot selector, selects active memory device.
+                All following accesses to other registers in 0xa00-0xa17
+                region will read/store data from/to selected memory device.
+      [0x4-0x7] OST event code reported by OSPM
+      [0x8-0xb] OST status code reported by OSPM
+      [0xc-0x13] reserved, writes into it are ignored
+      [0x14] Memory device control fields
+          bits:
+              0: reserved, OSPM must clear it before writing to register
+              1: if set to 1 clears device insert event, set by OSPM
+                 after it has emitted device check event for the
+                 selected memory device
+              2-7: reserved, OSPM must clear them before writing to register
+
+Selecting memory device slot beyond present range has no effect on platform:
+   - write accesses to memory hot-plug registers not documented above are
+     ignored
+   - read accesses to memory hot-plug registers not documented above return
+     all bits set to 1.