arm64: dts: marvell: armada-ap806: reserve PSCI area

The memory area [0x4000000-0x4200000[ is occupied by the PSCI firmware. Any
attempt to access it from Linux leads to an immediate crash.

So let's make the same memory reservation as the vendor kernel.

[gregory: added as comment that this region matches the mainline U-boot]
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>