serial: fix input interrupts on VMWare

The serial-port input was not working on VMWare, because interrupts were
not received. It turns out that VMWare does not send serial-port interrupts
(IRQ 4) unless the Aux Output 2 bit of the MCR register in the UART is
enabled. Many physical PCs also require this bit, but it's surprising that
VMWare bothers to check it - QEMU doesn't.

So this patch turns on that bit, and serial-port input starts to work
correctly on VMWare.

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>