soc: Drive uart1_irq to 0 when we don't have UART1

The tools complain about uart1_irq not being driven and not having a
default when HAS_UART1 is false.  This sets it to 0 in that case.

Fixes: 7575b1e0c2 ("uart: Import and hook up opencore 16550 compatible UART")
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>

soc.vhdl

@@ -754,6 +754,7 @@ begin
 	wb_uart1_out.dat <= x"00000000";
 	wb_uart1_out.ack <= wb_uart1_in.cyc and wb_uart1_in.stb;
 	wb_uart1_out.stall <= '0';
+        uart1_irq <= '0';
     end generate;
 
     spiflash_gen: if HAS_SPI_FLASH generate