feat(mt8186): initialize platform for MediaTek MT8186

- Add basic platform setup.
- Add MT8186 documentation at docs/plat/.
- Add generic CPU helper functions.
- Add basic register address.

TEST=build pass
BUG=b:202871018

Change-Id: Id3e2f46a8c3ab2f3e29137e508d4c671e8f4aad5
diff --git a/docs/plat/index.rst b/docs/plat/index.rst
index 5848005..eadd946 100644
--- a/docs/plat/index.rst
+++ b/docs/plat/index.rst
@@ -20,6 +20,7 @@
    intel-stratix10
    marvell/index
    mt8183
+   mt8186
    mt8192
    mt8195
    nvidia-tegra
diff --git a/docs/plat/mt8186.rst b/docs/plat/mt8186.rst
new file mode 100644
index 0000000..16b833a
--- /dev/null
+++ b/docs/plat/mt8186.rst
@@ -0,0 +1,21 @@
+MediaTek 8186
+=============
+
+MediaTek 8186 (MT8186) is a 64-bit ARM SoC introduced by MediaTek in 2021.
+The chip incorporates eight cores - six Cortex-A55 little cores and two Cortex-A76.
+Cortex-A76 can operate at up to 2.05 GHz.
+Cortex-A55 can operate at up to 2.0 GHz.
+
+Boot Sequence
+-------------
+
+::
+
+    Boot Rom --> Coreboot --> TF-A BL31 --> Depthcharge --> Linux Kernel
+
+How to Build
+------------
+
+.. code:: shell
+
+    make CROSS_COMPILE=aarch64-linux-gnu- PLAT=mt8186 DEBUG=1 COREBOOT=1