generalize renesas LINK core driver

create local register access struct and move mcu specific code
	in preparation of support for other mcu families that use the LINK usb core

Signed-off-by: Rafael Silva <rafaelsilva@ajtec.pt>

hw/bsp/rx/boards/rx65n_target/rx65n_target.c

@@ -248,6 +248,11 @@ void board_init(void)
   IEN(ICU,GROUPBL0)  = 1;
   EN(SCI5, TEI5)     = 1;
 
+  /* Enable USB0 */
+  SYSTEM.PRCR.WORD = SYSTEM_PRCR_PRKEY | SYSTEM_PRCR_PRC1;
+  MSTP(USB0) = 0;
+  SYSTEM.PRCR.WORD = SYSTEM_PRCR_PRKEY;
+
   /* setup USBI0 interrupt. */
   IR(USB0, USBI0)  = 0;
   IPR(USB0, USBI0) = IRQ_PRIORITY_USBI0;