irqs should only be requested/released with enabled DMA. Previously
interrupt statistics where disabled for IMX Processors because of
different writeable SISR bits.

This patch introduces support for irqstats on imx processors again by
creating a sisr writeback mask and introducing a imx35-ssi compatible.
Signed-off-by: NMarkus Pargmann <mpa@pengutronix.de>
Signed-off-by: NMark Brown <broonie@linaro.org>