boards: arm: Add support for R-Car H3 Salvator-X board

Add initial support for the Salvator-X development board.

This work is strongly based on the H3ULCB board, but compared to it:
- device tree was enhanced
   - adding all available push buttons and LEDs
   - setting proper names according to the refdes used on the schematic and
     PCB silkscreen
- documentation was adapted to this board

Signed-off-by: Valerio Setti <vsetti@baylibre.com>