qemu/qemu_sbsa: enable secure variable storage

qemu/qemu_sbsa: enable secure variable storage

This implements support for UEFI secure variable storage
using standalone MM framework on qemu_sbsa platform.

Non-secure shared memory between UEFI and standalone MM
is allocated at the top of DRAM.
DRAM size of qemu_sbsa varies depends on the QEMU parameter,
so the non-secure shared memory is allocated by trusted firmware
and passed the base address and size to UEFI through device tree
"/reserved-memory" node.

Change-Id: I367191f408eb9850b7ec7761ee346b014c539767
Signed-off-by: Masahisa Kojima <masahisa.kojima@linaro.org>