novena: Remove empty reset_cpu()

There is really no need to provide an empty reset_cpu() implementation,
so just remove it.

Cc: Marek Vasut <marex@denx.de>
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Marek Vasut <marex@denx.de>