am33xx, spl, siemens: enable debug uart output again

a6b541b0: TI ARMv7: Don't use GD before crt0.S has set it

moves the init of the debug uart at the very end of SPL code.
Enable it for the siemens board earlier, as they print
ddr settings ... all debug output before board_init_r()
is here currently useless. Maybe we must rework this
globally?

Signed-off-by: Heiko Schocher <hs@denx.de>