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/plat/mediatek/mt8186/aarch64/plat_helpers.S b/plat/mediatek/mt8186/aarch64/plat_helpers.S
new file mode 100644
index 0000000..35b293f
--- /dev/null
+++ b/plat/mediatek/mt8186/aarch64/plat_helpers.S
@@ -0,0 +1,45 @@
+/*
+ * Copyright (c) 2021, ARM Limited and Contributors. All rights reserved.
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+
+#include <arch.h>
+#include <asm_macros.S>
+#include <platform_def.h>
+
+	.globl plat_is_my_cpu_primary
+	.globl plat_my_core_pos
+	.globl plat_mediatek_calc_core_pos
+
+func plat_is_my_cpu_primary
+	mrs	x0, mpidr_el1
+	and	x0, x0, #(MPIDR_CLUSTER_MASK | MPIDR_CPU_MASK)
+	cmp	x0, #PLAT_PRIMARY_CPU
+	cset	x0, eq
+	ret
+endfunc plat_is_my_cpu_primary
+
+	/* -----------------------------------------------------
+	 *  unsigned int plat_my_core_pos(void)
+	 *  This function uses the plat_mediatek_calc_core_pos()
+	 *  definition to get the index of the calling CPU.
+	 * -----------------------------------------------------
+	 */
+func plat_my_core_pos
+	mrs	x0, mpidr_el1
+	b	plat_mediatek_calc_core_pos
+endfunc plat_my_core_pos
+
+	/* -----------------------------------------------------
+	 * unsigned int plat_mediatek_calc_core_pos(u_register_t mpidr);
+	 *
+	 * With this function: CorePos = CoreID (AFF1)
+	 * we do it with x0 = (x0 >> 8) & 0xff
+	 * -----------------------------------------------------
+	 */
+func plat_mediatek_calc_core_pos
+	mov	x1, #MPIDR_AFFLVL_MASK
+	and	x0, x1, x0, lsr #MPIDR_AFF1_SHIFT
+	ret
+endfunc plat_mediatek_calc_core_pos