ARM: at91: usart: drop static map regs for dbgu

In commit fb149f9e28354 we introduce ioremap support for static map_io, we do
not need this register entry anymore.

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