how to increase the depth of dcfifo more than 131000 words ?

Using up all the on chip RAM when you have external RAM seems like a waste of resources. Just build an ADC interface that supports Avalon-ST, hook it up to a clock crossing FIFO if need, then hook up the FIFO/ADC to a DMA and push it into memory. Also long as you have enough memory bandwidth and you feed the DMA descriptors fast enough you should never drop samples.