xilinx_spips: Fix CTRL register RW bits

The CTRL register was RAZ/WI on some of the RW bits. Even though the
function behind these bits is invalid in QEMU, they should still be
guest accessible. Fix.

Signed-off-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@gmail.com>
Message-id: b7aaad93163ce4af0c428635804ac7b77a567b25.1369117359.git.peter.crosthwaite@xilinx.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
