mips/malta: fix CBUS UART interrupt pin

mips/malta: fix CBUS UART interrupt pin

According to the MIPS Malta Developement Platform User's Manual, the
i8259 interrupt controller is supposed to be connected to the hardware
IRQ0, and the CBUS UART to the hardware interrupt 2.

In QEMU they are both connected to hardware interrupt 0, the CBUS UART
interrupt being wrong. This patch fixes that. It should be noted that
the irq array in QEMU includes the software interrupts, hence
env->irq[2] is the first hardware interrupt.

Cc: Ralf Baechle <ralf@linux-mips.org>
Reviewed-by: Eric Johnson <ericj@mips.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>