ARM: at91: debug: use DEBUG_UART_VIRT

AT91 still uses an offset (0x0100 0000) from the physical address to map
the debug UART. This is unfortunate as for some platforms (sama5d3 and
earlier), it ends up in the PCI zone and PCI is enabled in multi_v7.
Switch to DEBUG_UART_VIRT to solve that.

Tested on sama5d3 and 9g20.

Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>