mpc83xx: spd_sdram.c: Disable memory controller before initializing

mpc83xx: spd_sdram.c: Disable memory controller before initializing

The memory controller could already be enabled, when spd_sdram() is
called. This could be the case for example, when the SDRAM is initialized
by the JTAG debugger.

The "sync" after the register access via the accessor function is
still needed, because the macro uses the sync before the real write
is done. So until not all accesses are converted to using accessor
functions, this sync still needs to be made "manually" here.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Reinhard Arlt <reinhard.arlt@esd.eu>
Acked-by: Dave Liu <daveliu@freescale.com>
Signed-off-by: Kim Phillips <kim.phillips@freescale.com>