arm/imx/gpio: GPIO_INT_{HIGH,LOW}_LEV are not necessarily constant

GPIO_INT_LOW_LEV is defined as

	(cpu_is_mx1_mx2() ? 0x3 : 0x0)

so depending on compiler optimisation and enabled SoCs this doesn't
qualify as a constant expression as needed by a switch statement.
Ditto for GPIO_INT_HIGH_LEV.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>