ARM: at91: remove empty at91_init_serial function

The real function is long gone and the empty one will generate warnings
when configured without Atmel serial:
  arch/arm/mach-at91/at91rm9200_devices.c:1176: warning: 'struct at91_uart_config' declared inside parameter list
  arch/arm/mach-at91/at91rm9200_devices.c:1176: warning: its scope is only this definition or declaration, which is probably not what you want

Signed-off-by: Joachim Eastwood <joachim.eastwood@jotron.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>