nios2: Replace fdt_translate_address with of_flat_dt_translate_address

nios2 builds fail with the following build error.

arch/nios2/kernel/prom.c: In function 'early_init_dt_scan_serial':
arch/nios2/kernel/prom.c:100:2: error:
	implicit declaration of function 'fdt_translate_address'

Commit c90fe9c0 ("of: earlycon: Move address translation to
of_setup_earlycon()") replaced fdt_translate_address() with
of_flat_dt_translate_address() but missed updating the nios2 code.

Fixes: c90fe9c0

 ("of: earlycon: Move address translation to of_setup_earlycon()")
Cc: Peter Hurley <peter@hurleysoftware.com>
Cc: Rob Herring <robh@kernel.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Acked-by: Ley Foon Tan <lftan@altera.com>