boards: nucleo_u5a5zj_q: add USB HS 2.0 support

This board has the required clock crystal (X4) and jumper settings
present to enable the USB 2.0 HS support.

* Enable the HSE clock (16MHz)
* Flip the PLL1 configuration over to use the HSE clock, but still
  outputting 160MHz to sysclk/apbclk.
* Add the USB HS device tree node.
* Update the board documentation.

Signed-off-by: Adrian Chadd <adrian.chadd@meta.com>