dt-bindings: reset: add sama7g5 definitions

Upstream linux commit 5994f58977e0.

Add reset bindings for SAMA7G5. At the moment only USB PHYs are
included.

The three reset USB phy's have their ID's mapped from 4 to 6. There are
no USB phy's with ID's numbered from 0 to 3.

Signed-off-by: Sergiu Moga <sergiu.moga@microchip.com>