ARM: at91: fix Kconfig.debug by adding DEBUG_AT91_UART option

The DEBUG_AT91_UART Kconfig option was forgotten when moving the
AT91 debug-macro.S file. Add it and use it for the at91.S compilation.

Reported-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>