spl, spl_nor: fix compiler warning

executing "tools/buildman/buildman mpc5xx" drops this warning:

common/spl/spl_nor.c: In function 'spl_nor_load_image':
common/spl/spl_nor.c:26:10: warning: assignment discards 'const' qualifier from pointer target type [enabled by default]

fix this.

Signed-off-by: Heiko Schocher <hs@denx.de>