ARM: dts: at91: sama7g5ek: Align the impedance of the QSPI0's HSIO and PCB lines

The impedance of the QSPI PCB lines on the sama7g5ek is 50 Ohms.
Align the output impedance of the QSPI0 HSIOs by setting a medium drive
strength which corresponds to an impedance of 56 Ohms when VDD is in the
3.0V - 3.6V range. The high drive strength setting corresponds to an
output impedance of 42 Ohms on the QSPI0 HSIOs.
This is just a fine tunning. The memory that we have populated on sama7g5ek
works fine even with high drive strength, but it's better to adjust it and
use medium instead, in case some other flashes with higher frequencies are
tested.

Suggested-by: Mihai Sain <mihai.sain@microchip.com>
Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>