mpc83xx: mpc8360emds: Don't use LBC SDRAM when DDR is available

Since commit 5c2ff323 ("mpc8360emds:
rework LBC SDRAM setup"), LBC SDRAM is available for use in Linux.

Though, it appears that QE Ethernet in Gigabit mode can't transmit
large packets when it tries to work with a data in LBC SDRAM (memtest
didn't discover any issues, is LBC SDRAM just too slow?).

With this patch we can still use the board without DDR memory, but
if DDR is available, we don't use LBC SDRAM.

Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Signed-off-by: Kim Phillips <kim.phillips@freescale.com>