UBUNTU: d-i initrd needs additional usb modules to support the merlin platform

BugLink: https://launchpad.net/bugs/1625222

Ubuntu builds the uas and xhci-plat-hcd usb drivers as modules, these
module are needed for d-i to use usb in the installer on the merlin platform.
Signed-off-by: Craig Magina <craig.magina@canonical.com>
Acked-by: Tim Gardner <tim.gardner@canonical.com>
Acked-by: Seth Forshee <seth.forshee@canonical.com>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

debian.master/d-i/modules/usb-modules

@@ -10,3 +10,5 @@ uhci-hcd ?
 xhci-hcd ?
 ehci-tegra ?
 ehci-msm ?
+uas ?
+xhci-plat-hcd ?