Allow attaching devices to OMAP UARTs.

Also avoid two signedness warnings in hw/omap2.c.
The API to attach new devices to serials is fine, bu the implementation
is a hack.


git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@5263 c046a42c-6fe2-441c-8c8c-71466251a162
