UPSTREAM: xhci: prepare for operation without shared HCD

This patch is reworked as multiple patches went to support target
with only one roothub.
This patch prepares xhci for the following scenario:
- If either of the root hubs has no ports, then omit shared HCD.
- The main HCD can be USB3 if there are no USB2 ports.

(cherry picked from commit
57f23cd0bf2f ("xhci: factor out parts of xhci_gen_setup().")
4a593a62a9e3a (BACKPORT: xhci: Fix null pointer dereference in removal
 if xHC has only one roothub.")
669bc5a188b40 ("UPSTREAM: xhci: Add bus number to some debug messages.")
873f323618c20 ("UPSTREAM: xhci: prepare for operation without shared HCD.)"
0cf1ea040a7e2 ("BACKPORT: usb: host: xhci-plat: create shared HCD
 after having added the main HCD.")
e0fe986972f5b ("BACKPORT: usb: host: xhci-plat: prepare operation without
 shared HCD.")
4736ebd7fcaff ("UPSTREAM: usb: host: xhci-plat: omit shared HCD if
 either root hub has no ports.")
1bd8bb7d2dfc4 ("xhci: Don't defer primary roothub registration if
 there is only one roothub.")
https: //git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
master).

Change-Id: I2ebdb15ebc2125db6ee18f14291f5590139adbdf
Signed-off-by: Akash Kumar <quic_akakum@quicinc.com>