[FFA] Move mu_silicon_arm and mu_tiano to mainline (#1122)

## Description

This change picks up the changes moved to mainline of mu_silicon_arm and
mu_tiano.

Also integrate the changes from mu_feature_ffa branch.

For details on how to complete these options and their meaning refer to
[CONTRIBUTING.md](https://github.com/microsoft/mu/blob/HEAD/CONTRIBUTING.md).

- [ ] Impacts functionality?
- [ ] Impacts security?
- [x] Breaking change?
- [ ] Includes tests?
- [ ] Includes documentation?

## How This Was Tested

This change is tested on QEMU SBSA platform.

## Integration Instructions

N/A

Author: kuqin12

.gitmodules

@@ -38,3 +38,6 @@
 [submodule "Silicon/Arm/HAF"]
 	path = Silicon/Arm/HAF
 	url = https://github.com/kuqin12/hafnium.git
+[submodule "Features/FFA"]
+	path = Features/FFA
+	url = https://github.com/microsoft/mu_feature_ffa.git