nios2: Use the generic bitops headers

The generic bitops headers are required when calling logarithmic
functions, such as ilog2().

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Thomas Chou <thomas@wytron.com.tw>
Reviewed-by: Jagan Teki <jteki@openedev.com>