ARM: dts: stm32: enable USB Host (USBH) EHCI controller on stm32mp15 DKx

This patch enables USB Host (USBH) EHCI controller on stm32mp15 dk boards.
As a hub is used between USBH and USB connectors, no need to enable
USBH OHCI controller: all low- and full-speed traffic is managed by the
hub.

Signed-off-by: Amelie Delaunay <amelie.delaunay@st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>