spi: spi-fsl-dspi: use IRQF_SHARED mode to request IRQ

Some SoC share one irq number between DSPI controllers.
For example, on the LX2160 board, DSPI0 and DSPI1 share one irq number.
In this case, only one DSPI controller can register successfully,
and others will fail.

Signed-off-by: Chuanhua Han <chuanhua.han@nxp.com>
Signed-off-by: Mark Brown <broonie@kernel.org>