allwinner: Relax PSCI entry point check

The DRAM controller supports up to 4GB of DRAM, and there are actually
boards out there where we can use at least 3GB of this.

Relax the PSCI entry point check, to be not restricted to 2GB of DRAM.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>