SHANGHAI, May 24 (Reuters) - China stocks eked out gains on Monday, helped by gains for financial firms, but caution prevailed as investors awaited key U.S. inflation readings for guidance on monetary policy. ** The CSI300 index rose 0.1% to 5,137.63 points at the end of the morning session, while the Shanghai Composite Index gained 0.2% to 3,492.04 points. ** Leading the gains, the CSI SWS securities index jumped 3%, helping push up the CSI300 financials index ...