Move CONFIG_USB_XHCI to defconfig files for all boards, renaming it
into CONFIG_USB_XHCI_HCD.

As commented in the help of "config USB_XHCI" entry, this has been
a TODO for a long time; now CONFIG_USB_XHCI_HCD and CONFIG_USB_XHCI
have been unified in favor of the former.

Note:
Some boards define CONFIG_USB_XHCI in their headers without
CONFIG_USB, which does not meet the "depends on" in Kconfig.
I added CONFIG_USB=y for those boards when converting.
Otherwise, they would fail to build.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>