ARM: at91: make DBGU soc independent

we will select now the DBGU used by the soc at Kconfig level

For the DEBUG_LL and early_printk this will allow to select which DBGU to use
this will also allow to select them when multiple SOC are enabled

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>