board: st: stm32f746-disco: fix console is not enabled while init dram

While initializing dram in spl_dram_init(), mdelay() is called that in
order calls get_ticks() that  verifies if timer exists, if doesn't, it
throws a panic(), but since preloader_console_init() has still not been
called those panic()s will fail. This doesn't help debugging, so let's
setup console before calling spl_dram_init() by moving it after
spl_dram_init().

Signed-off-by: Giulio Benetti <giulio.benetti@benettiengineering.com>