Fix UART register map

While it's unused upstream, according to the SiFive FU540 document, the
UART divisor register is at offset 0x18.

This also maps the interrupt enable and interrupt pending register
offsets.

machine/uart.h

@@ -11,7 +11,9 @@ extern volatile uint32_t* uart;
 #define UART_REG_RXFIFO		1
 #define UART_REG_TXCTRL		2
 #define UART_REG_RXCTRL		3
-#define UART_REG_DIV		4
+#define UART_REG_IE			4
+#define UART_REG_IP			5
+#define UART_REG_DIV			6
 
 #define UART_TXEN		 0x1
 #define UART_RXEN		 0x1