mpc83xx: spd_sdram: fix ddr sdram base address assignment bug

The spd_dram code shifts the base address, then masks 20 bits, but
forgets to shift the base address back. Fix this by just masking the
base address correctly.

Found this bug while trying to relocate a DDR memory at the base != 0.

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