ARM: at91: avoid defining CONFIG_* symbols in source code

In an invalid randconfig build (fixed by another patch),
I ran across this warning:

arch/arm/include/debug/at91.S:18:0: error: "CONFIG_DEBUG_UART_VIRT" redefined [-Werror]
 #define CONFIG_DEBUG_UART_VIRT AT91_IO_P2V(CONFIG_DEBUG_UART_PHYS)

As Russell pointed out, we should never #define a macro starting
with CONFIG_ in a source file, as that is rather confusing.

This renames the macro to avoid the symbol clash.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Suggested-by: Russell King <linux@arm.linux.org.uk>
Acked-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>