ARM: imx: drop support for irq priorisation

The code for irq priorisation support doesn't have any in-tree users and
the Kconfig description does wrong promises because nowadays irq
handlers are called with irqs disabled, so no high prioritized irq can
interrupt a lower prioritized handler.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>