PPC4xx: Correct SDRAM_MCSTAT for PPC405EX(r)

While the PowerPC 405EX(r) shares in common the AMCC/IBM DDR2 SDRAM
controller core also used in the 440SP, 440SPe, 460EX, and 460GT, in
the 405EX(r), SDRAM_MCSTAT has a different DCR value.

Its present value on the 405EX(r) causes a read back of 0xFFFFFFFF
which causes SDRAM initialization to periodically fail since it can
prematurely indicate SDRAM ready status.

Signed-off-by: Grant Erickson <gerickson@nuovations.com>
Signed-off-by: Stefan Roese <sr@denx.de>