arm/imx-common: Fix warning 'get_reset_cause' defined but not used

Fix below warning
arch/arm/imx-common/cpu.c:29:14: warning: ‘get_reset_cause’ defined but
not used
 static char *get_reset_cause(void)

Signed-off-by: Prabhakar Kushwaha <prabhakar@freescale.com>
Reviewed-by: Eric Nelson <eric.nelson@boundarydevices.com>
Acked-by: Stefano Babic <sbabic@denx.de>