qemu conf: enable ION for OP-TEE/SDP test support

Enable Linux kernel ION, ION_DUMMY.
Statically define an 3MByte unmapped heap in ION dummy driver. This
complies with the OP-TEE layout for qemu_virt and qemu_armv8 platforms.

CONFIG_ION has dependencies on CONFIG_ANDROID. This changes enables
ION from the Android staging drivers but without enabling other
staged Android features (TIMED_OUTPUT and LOW_MEMORY_KILLER).

Signed-off-by: Etienne Carriere <etienne.carriere@linaro.org>
Reviewed-by: Joakim Bech <joakim.bech@linaro.org>