cadence_ttc: Fix 'clear on read' behavior

cadence_ttc: Fix 'clear on read' behavior

A missing call to qemu_set_irq() when reading the IRQ register
required SW to write to the IRQ register to acknowledge an
interrupt. With this patch the behavior is fixed:
 - Reading the interrupt register clears it and updates the timers
   interrupt status
 - Writes to the interrupt register are ignored

Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Signed-off-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>