xilinx_spips: Add missing dual-bus snoop commands

Added additional commands to the switch to check for when snooping commands in
dual bus mode setups. Cleaned up code to use an enum.

Signed-off-by: Nathan Rossi <nathan.rossi@xilinx.com>
Signed-off-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Message-id: 848c116c711dab0af10729a487968384aadd9faf.1362373359.git.peter.crosthwaite@xilinx.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
