powerpc/8xxx: Fix compile warning when build for a DDR1 or DDR2 board

ctrl_regs.c: In function 'set_ddr_sdram_mode_2':
ctrl_regs.c:690:6: warning: unused variable 'i'

'i' is only used by DDR3 code.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>