ddr: fsl: Remove CONFIG_MEM_INIT_VALUE

The way all of the memory init code here works is that we pass
0xDEADBEEF around for the initial value (as it's a well known 'poison'
value and so easily recognized in debuggers, etc). The only point of
this CONFIG symbol was to pass in a different value for that purpose.
Drop this symbol and cleanup the code slightly.

Signed-off-by: Tom Rini <trini@konsulko.com>