usb: xhci: move xhci.h to include usb

The xhci.h header file is currently located under drivers/usb/xhci
Move it to the include/usb folder to make it available to drivers that
are not under drivers/usb/xhci

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
diff --git a/MAINTAINERS b/MAINTAINERS
index a7c355c..48345f8 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -851,6 +851,7 @@
 S:	Maintained
 T:	git https://gitlab.denx.de/u-boot/custodians/u-boot-usb.git topic-xhci
 F:	drivers/usb/host/xhci*
+F:	include/usb/xhci.h
 
 VIDEO
 M:	Anatolij Gustschin <agust@denx.de>