verification/rvgen: Support the 'next' operator

The 'next' operator is a unary operator. It is defined as: "next time, the
operand must be true".

Support this operator. For RV monitors, "next time" means the next
invocation of ltl_validate().

Cc: John Ogness <john.ogness@linutronix.de>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Link: https://lore.kernel.org/9c32cec04dd18d2e956fddd84b0e0a2503daa75a.1752239482.git.namcao@linutronix.de


Signed-off-by: Nam Cao <namcao@linutronix.de>
Tested-by: Gabriele Monaco <gmonaco@redhat.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>