projects:ad405x_iio:Add support for large capture

[ribdp]

1. Remove DMA double buffering
2. Use ping pong implementation where DMA always writes to SRAM buffer
3. Increase the size of SRAM IIO buffer to 128KB

[jansunil]

LGTM. Now that we have SDRAM support in the STM32 FW also, should this be updated in the IIO Osc documentation?

[ribdp]

Updated data_streaming.rst to not refer exclusively to Mbed firmware, when talking about enabling SDRAM