ARM: at91: debug: use DEBUG_UART_PHYS

Instead of having to add a new configuration option each time support for
new SoC is added, use CONFIG_DEBUG_UART_PHYS. For now,
CONFIG_DEBUG_UART_VIRT is automatically computed.

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