Styx/PlatformFlashAccessLib: Add progress API

https://bugzilla.tianocore.org/show_bug.cgi?id=801



Add PerformFlashWriteWithProgress() to the PlatformFlashAccessLib.
This allows the platform to inform the user of progress when a
firmware storage device is being updated with a new firmware
image.

This is the minimal update to this library implementation to
keep everything building and preserve any existing progress
indication.  Additional updates are required to use the
Progress() API passed into PerformFlashWriteWithProgress().

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Leif Lindholm <leif.lindholm@linaro.org>
Signed-off-by: Michael D Kinney <michael.d.kinney@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>