global_data: Add a generic global_data flag for SMP state

Allow keeping track of whether all CPUs have been enabled yet. This allows
us to know whether other CPUs need to be considered when updating
CPU-specific settings such as MTRRs on x86.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Wolfgang Wallner <wolfgang.wallner@br-automation.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>