Add USB support to OMAP3 model

At the moment the OMAP3/beagle model doesn't have working USB. This is important because for beagle keyboard, mouse and networking are all USB devices. The situation is slightly complicated because currently QEMU doesn't support EHCI USB, only OHCI (EHCI support is being worked on but there is no ETA for it). Beagle hardware has (1) USB OTG and (2) EHCI USB (although OMAP3 itself has both OHCI and EHCI). The goal here is a beagle model which can do keyboard, mouse, networking, plus documentation on the wiki of any necessary config/command line options. The Linux omap3 kernel doesn't try to use the OHCI hardware at all, only EHCI and the USB OTG.

Update: it looks as if EHCI support may be landing in QEMU in the not-too-distant future, although it is likely to be unstable initially.

The OTG support was at one point disabled in the kernel, but in linaro's 1105 beta release at least it seems to be back. USB OTG is therefore the most plausible route forwards. Recent Linaro omap kernels also seem to look at the OHCI controller, so this might also be worth modelling properly.