Introduce basic USB related header files.

[MichalBlk]

The presented header files are required by #935.
Presented changes (regarding original files):

• scsi.h:
  • length and addr fields aren't expressed in byte units (i.e. instead of uint8_t addr[4] we have uint32_t addr). This simplifies little endian <-> big endian conversions.
  • I've added the SID_FORMAT() macro (used in drive identification).
• uhci.h: this is simply a subset of the original file (with a few stylistic changes).
• uhcireg.h:
  • I've added PCI_USB_CLASSCODE and PCI_USB_SUBCLASSCODE.
  • I've added UHCI_PORTSC() instead of UHCI_PORTSC{1,2} (The UHCI specification allows more than two root hub ports).
  • I've added UHCI_PORTSC_ONE which is used in root hub port detection (I don't assume any specific number of root hub ports).
• umass.h: this file doesn't have a strict counterpart in the FreeBSD source tree. Contained definitions were extracted form the FreeBSD's sys/dev/usb/storage/umass.c file. This is in order to make the umass.c driver thinner and to contain some definitions required by the USB module (UR_BBB_RESET and UR_BBB_GET_MAX_LUN).
• usbhid.h: this is simply a subset of the original file (without the HID descriptor parsing definitions (we assume the boot protocol is supported)).
• usb.h:
  • I've added the UV_MAKE() macro (they use some counterpart defined elsewhere).
  • I've added the US_DATASIZE constant (they use a magic number).
  • I've added the US_ENG_LID and US_ENG_STR constants.
  • I've added the US_TRANSFER_TYPE() macro.

[cahirwpz]

All remarks for scsi.h file apply to other header files as well.

[MichalBlk]

I've described the presented changes to the original files (see the PR's description).