ARM: dts: at91: rename sama7g5ek.dts to at91-sama7g5ek.dts

In Linux this DT file is named at91-sama7g5ek.dts. Rename it accordingly.

Signed-off-by: Eugen Hristev <eugen.hristev@microchip.com>
Reviewed-by: Tudor Ambarus <tudor.ambarus@microchip.com>