mmc: dw_mmc: Convert to use MMC_CAP2_SDIO_IRQ_NOTHREAD for SDIO IRQs

Convert to use the more lightweight method for processing SDIO IRQs, which
involves the following changes:

- Enable MMC_CAP2_SDIO_IRQ_NOTHREAD when SDIO IRQ is supported and use
  sdio_signal_irq() instead of mmc_signal_sdio_irq().
- Mask the SDIO IRQ before signaling a new one to be processed.
- Implement the ->ack_sdio_irq() callback to unmask the SDIO IRQ.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Tested-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>