arcade: trigger nvram save from core.

user_io.cpp

@@ -3176,6 +3176,7 @@ void user_io_poll()
 
 	if (is_megacd()) mcd_poll();
 	if (is_pce()) pcecd_poll();
+	if (is_arcade() && spi_uio_cmd(UIO_CHK_UPLOAD)) arcade_nvm_save();
 	process_ss(0);
 }
 

user_io.h

@@ -67,6 +67,7 @@
 #define UIO_SET_AFILTER 0x39
 #define UIO_SET_AR_CUST 0x3A
 #define UIO_SET_UART    0x3B
+#define UIO_CHK_UPLOAD  0x3C
 
 // codes as used by 8bit for file loading from OSD
 #define FIO_FILE_TX     0x53