Add support for USB3.0 on Armada38x

This commit provides attachment of xhci-platform for A38X boards, making
it possible to mount FreeBSD world from USB3.0 flash. 'xhci' device was
added to files.mv (as optional) and kernconf of Armada38x was enhanced.

It was also necessary to open programmable memory windows of USB3.0.
fdt_win_setup needed improvement so it's able to traverse through
children of internal-regs node.

Submitted by: Bartosz Szczepanek <bsz@semihalf.com>
Obtained from: Semihalf
Sponsored by: Stormshield
Reviewed by: hselasky
Approved by: cognet (mentor)
Differential Revision: https://reviews.freebsd.org/D5031