sections.py: Added top_module to VerilatorSection
verilator: add top_module parameter

This patch adds top_module parameter to the verilator
CAPI section. This parameter is mandatory for the
system core.

It also make tb_toplevel not optional.

Signed-off-by: Franck Jullien <franck.jullien@gmail.com>

Conflicts:
	fusesoc/simulator/verilator.py