imx: cpu.c: give access to reset cause in spl

This makes get_imx_reset_cause() accessible in SPL, but keeps the SRSR
register content intact so that U-Boot proper can evaluated the
reset_cause again should this be needed.

Signed-off-by: Max Krummenacher <max.krummenacher@toradex.com>
Acked-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>