MX31: Make get_reset_cause() static and drop unreachable code

get_reset_cause() should not be exported. Drop code in the function
after return statement that can generate warnings due to unreachable code.

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