ddr: altera: Add DDR2 support to Gen5 driver

Add DDR2 support to Gen5 DRAM driver. As the DDR2 macro names generated
by Quartus are named differently than the DDR3 ones, use anon unions to
store them in the same structures, without growing their size.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Ley Foon Tan <ley.foon.tan@intel.com>
Cc: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>