powerpc/83xx: fix sdram initialization for keymile boards

commit b11f53f3 (keymile: Fix Coding style issues for keymile boards)
introduces a bug according the SDRAM initialization for all
km83xx boards.

im->ddr.sdram_cfg |= SDRAM_CFG_MEM_EN;
was replaced with
out_be32(&im->ddr.sdram_cfg, SDRAM_CFG_MEM_EN);
and this is wrong, because this overwrites the intial value
CONFIG_SYS_DDR_SDRAM_CFG.

Signed-off-by: Andreas Huber <andreas.huber@keymile.com>
Signed-off-by: Holger Brunck <holger.brunck@keymile.com>
Acked-by: Heiko Schocher <hs@denx.de>
Signed-off-by: Kim Phillips <kim.phillips@freescale.com>