c6x: asm: Add default flat.h according to xtensa architecture

For supporting uClinux flat-format executables, c6x needs to define the
fewest features to support it, at present, xtensa architecture has the
fewest feature for it, so just copy xtensa flat.h. The related error:

    CC      fs/binfmt_flat.o
  In file included from fs/binfmt_flat.c:36:0:
  include/linux/flat.h:12:22: fatal error: asm/flat.h: No such file or directory
Signed-off-by: NChen Gang <gang.chen.5i5j@gmail.com>
Signed-off-by: NMark Salter <msalter@redhat.com>

arch/c6x/include/asm/flat.h

+#ifndef __ASM_C6X_FLAT_H
+#define __ASM_C6X_FLAT_H
+
+#define flat_argvp_envp_on_stack()			0
+#define flat_old_ram_flag(flags)			(flags)
+#define flat_reloc_valid(reloc, size)			((reloc) <= (size))
+#define flat_get_addr_from_rp(rp, relval, flags, p)	get_unaligned(rp)
+#define flat_put_addr_at_rp(rp, val, relval)		put_unaligned(val, rp)
+#define flat_get_relocate_addr(rel)			(rel)
+#define flat_set_persistent(relval, p)			0
+
+#endif /* __ASM_C6X_FLAT_H */