hw/omap: Fix default setup for OMAP UART devices

hw/omap: Fix default setup for OMAP UART devices

Character devices created by qemu_chr_open don't
allow duplicate device names, so naming all
UART devices "null" no longer works.

Running "qemu-system-arm -M n800" (and some other machines)
results in this error message:

	qemu-system-arm: Duplicate ID 'null' for chardev
	Can't create serial device, empty char device

This is fixed by setting a default label "uart1",
"uart2" or "uart3".

Cc: Andrzej Zaborowski <[email protected]>
Signed-off-by: Stefan Weil <[email protected]>