xilinx_spips: Trash LQ page cache on mode change

Invalidate the LQSPI cached page when transitioning into LQSPI mode.
Otherwise there is a possibility that the controller will return stale
data to the guest when transitioning back to LQ_MODE after a page
program.

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: 677490a6ee1953fe5d366e599d665de645ac84db.1369117359.git.peter.crosthwaite@xilinx.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
