platform: generic: thead: move to thead c9xx header to vendor specific postion

The CSR encoding for t-head c9xx cores is shared across all the
platforms with these cores. So move header thead_c9xx.h to the
thead subdir.

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