[SERIAL] Small time UART configuration fix for AU1100 processor

The AU1100 processor does not have an internal UART2. Only
UART0, UART1 and UART3 exist. This patch removes the non existing
UART2 and replaces it with a descriptive comment.

Signed-off-by: Freddy Spierenburg <freddy@dusktilldawn.nl>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>