Robinhood, the U.S. online broker that has emerged as a gateway for amateur traders challenging Wall Street hedge funds, has held talks with banks about raising $1 billion in debt so it can continue to fulfill orders for heavily shorted stocks, according to people familiar with the matter. The capital raised would be separate from the $3.4 billion in financing that Robinhood announced on Monday it had secured from its investors since Jan. 29. It reflects the financial pressure that last week's Reddit-fueled frenzy in shares such as GameStop Corp placed on the company, prompting it to restrict some trades.