arm64: dts: marvell: armada-37xx: reserve memory for ATF

The PSCI area should be reserved in Linux for PSCI operations such as
suspend/resume.

Reserve 2MiB of memory which matches the area used by ATF (BL1, BL2,
BL3x, see [1] in ATF source code). This covers all PSCI code and data
area and is 2MiB aligned, which is required by Linux for huge pages
handling.

Please note that this is a default setup allowing to perform PSCI
operations with legacy bootloaders. Recent bootloaders should update the
region size/position accordingly.

[1] plat/marvell/a3700/common/include/platform_def.h

Signed-off-by: Victor Gu <xigu@marvell.com>
[miquel.raynal@bootlin.com: reword of commit message, comment in the DTSI]
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Acked-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>