acpi: arm/virt: build_spcr: fix invalid cast

implicit cast to structure uint8_t member didn't raise error when
assigning value from incorrect enum, but when using build_append_gas()
(next patch) it will error out with (clang):
  implicit conversion from enumeration type 'AmlRegionSpace'
  to different enumeration type 'AmlAddressSpace'
fix cast error by using correct AML_AS_SYSTEM_MEMORY enum

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Message-Id: <20210924122802.1455362-31-imammedo@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>