soc: microchip: sam: reorganize directory structure for sama7g5 series

Update directory structure to the following hierarchy for MPU devices:
  Product Architecture
    |__ SOC Series
          |__ Product Group
Move directory 'soc/microchip/sama7g5/' into 'soc/microchip/sama7/'.
Add sama7g5 to 'soc/microchip/sam/sama7/soc.yml'.
Remove the files under soc/microchip/sam due to they became useless
with the reorganization.

Signed-off-by: Tony Han <tony.han@microchip.com>