zmx25: Fix build warning due to 'get_reset_cause' defined but not used

When building the zmx25 target we get:

Configuring for zmx25 board...
generic.c:108: warning: 'get_reset_cause' defined but not used

Fix this warning by defining get_reset_cause only if CONFIG_DISPLAY_CPUINFO is defined.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Stefano Babic <sbabic@denx.de>