Blackfin: bf518f-ezbrd: fix SPI CS for SPI flash

The SPI flash on the BF518F-EZBRD board is actually hooked up to CS2,
not CS1, so make sure the resources are correct.

URL: http://blackfin.uclinux.org/gf/tracker/5220


Signed-off-by: Graf Yang <graf.yang@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>