platform: generic: thead: separate implement of T-HEAD c9xx pmu

Separate the implement of T-HEAD c9xx pmu to allow any platform with
c9xx cores can use it.

Signed-off-by: Inochi Amaoto <inochiama@outlook.com>
Reviewed-by: Anup Patel <anup@brainfault.org>

platform/generic/thead/Kconfig

@@ -0,0 +1,5 @@
+# SPDX-License-Identifier: BSD-2-Clause
+
+config THEAD_C9XX_PMU
+	bool "T-HEAD c9xx M-mode PMU support"
+	default n