Add UART interrupts

Currently an interrupt is triggered any time there is any activity on
the UART_RXD line, but later it will only trigger once there is data
ready to be read.

hw/fpga/uart.sv

@@ -12,9 +12,13 @@ module uart(
     
     output logic TXD,
     
+    output logic irq,
     output logic [7:0] data_out
 );
 
+//Temporary!
+assign irq = ~RXD;
+
 //Handle reading and writing registers
 
 logic [7:0] tx_buf;