am33xx:ddr:Fix config_sdram to work for all DDR

The original write to sdram_config is correct for DDR3 but incorrect
for DDR2 so SPL was hanging. For DDR2, the write to sdram_config
should be after the writes to ref_ctrl. This was working for DDR3
because there was a write of 0x2800 to ref_ctrl before a write
to sdram_config.

Tested on: GP EVM 1.1A (DDR2), GP EVM 1.5A (DDR3),
           Beaglebone A6 (DDR2), Beagleone Blacd A4A (DDR3)

Signed-off-by: Steve Kipisz <s-kipisz2@ti.com>