MIPS: SGI-IP27: restructure ioc3 register access

Break up the big ioc3 register struct into functional pieces to
make use in sub-function drivers more straightforward. And while
doing that get rid of all volatile access by using readX/writeX.
Signed-off-by: Thomas Bogendoerfer <tbogendoerfer@suse.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

arch/mips/sgi-ip27/ip27-console.c

@@ -35,6 +35,7 @@ void prom_putchar(char c)
 {
 	struct ioc3_uartregs *uart = console_uart();
 
-	while ((uart->iu_lsr & 0x20) == 0);
-	uart->iu_thr = c;
+	while ((readb(&uart->iu_lsr) & 0x20) == 0)
+		;
+	writeb(c, &uart->iu_thr);
 }