devkit8000/spl: init GPMC for dm9000 in SPL

Linux crashes if the GPMC isn't configured for the dm9000.

Signed-off-by: Simon Schwarz <simonschwarzcor@gmail.com>
CC: Tom Rini <tom.rini@gmail.com>
CC: Stefano Babic <sbabic@denx.de>
CC: Wolfgang Denk <wd@denx.de>