include/hw/boards.h: Document memory_region_allocate_system_memory()

Add a documentation comment for memory_region_allocate_system_memory().

In particular, the reason for this function's existence and the
requirement on board code to call it exactly once are non-obvious.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 1499438577-7674-2-git-send-email-peter.maydell@linaro.org