powerpc: Use the poison value of 0xdeadbeef directly in DDR init

On p1_p2_rdb_pc platforms, we set ddr_data_init to the "poison" value of
0xdeadbeef rather than a real calculated / derived value.  Do this
directly and comment rather than via CONFIG.

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