Add macros to enable and disable access to CP6.  On the iop3xx, enabling
CP6 access unfortunately also enables access to that coprocessor from
unprivileged code, so we need these macros to enable and disable access
to the coprocessor whenever we need to access it.
Signed-off-by: Lennert Buytenhek <buytenh@wantstofly.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>