boards: arduino: portenta_h7: Update clock tree to enable 400 MHz.

Previous clock frequencies were taken directly from the arduino example
and were limited to 96 MHz and 48 Mhz for the CM7 and CM4 respectively.
Updated the clock tree to take advantage of the faster clock speeds.

Introduced two board revisions as revision 1 of the portenta board does
not have the external LSE oscillator populated (default) and version 4.1
onwards does have the external oscillator populated. The user can choose
to build support for the required clock configuration using the revision
scheme built in to west. Updated the docs to reflect the same.

Signed-off-by: Rahul Arasikere <arasikere.rahul@gmail.com>