irqchip/irq-imx-gpcv2: Remove unused function

imx_gpcv2_get_wakeup_source() is not used anywhere, so remove it.

This fixes the following sparse warning:

drivers/irqchip/irq-imx-gpcv2.c:34:5: warning: symbol 'imx_gpcv2_get_wakeup_source' was not declared. Should it be static?

Fixes: e324c4dc

 ("irqchip/imx-gpcv2: IMX GPCv2 driver for wakeup sources")
Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>