arm/imx: fix irq_base for gpio

When gpio core dynamically allocate gpio number for a port, it starts
from the end of the total range, 0 ~ ARCH_NR_GPIOS.  That said, the
earlier a port gets probed, the bigger gpio number it gets assigned.
To match this, the irq_base for gpio should be assigned from
'MXC_GPIO_IRQ_START + ARCH_NR_GPIOS' decreasingly.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>