board_f: Drop the timer after relocation

Once U-Boot relocates itself the existing driver-model timer (if any) is
no-longer valid until the device is reinitialised. Any use of the device
may cause a crash. To handle this, set the timer to NULL after relocation.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>

common/board_f.c

@@ -952,6 +952,9 @@ void board_init_f_r(void)
 	 * UART if available.
 	 */
 	gd->flags &= ~GD_FLG_SERIAL_READY;
+#ifdef CONFIG_TIMER
+	gd->timer = NULL;
+#endif
 
 	/*
 	 * U-Boot has been copied into SDRAM, the BSS has been cleared etc.