dts: phy: Add clock-reference prop in stm32u5-otghs-phy

The OTG_HS PHY from stm32u5a5xx device require the correct reference
clock frequency selction in SYSCFG_OTGHSPHYCR. The current default is
hard coded to 16Mhz (which matches the development board crystal).
However, a custom board my require a different crystal and then the
USB will not work. This add a required field in the
st,stm32u5-otghs-phy binding to force user to select the correct
clock reference. The current nucleo_u5a5zj_q baord was updated to
reflect the mandatory field.

Signed-off-by: BUDKE Gerson Fernando <gerson.budke@leica-geosystems.com>