staging: vchiq_arm: add __user pointer annotations

Add __user to pointers based on context and suggestions by sparse.

Other sparse warnings still remain, in cases where more change would be
required apart from adding __user.

Signed-off-by: Szilveszter Székely <szekelyszilv@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>