arm64: mvebu: Restrict memory size to a usable maximum

Not all memory is mapped in the MMU. So we need to restrict the memory
size so that U-Boot does not try to access it. Also, the internal
registers are located at 0xf000.0000 - 0xffff.ffff. Currently only 2GiB
are mapped for system memory. This is what we pass to the U-Boot
subsystem here.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Nadav Haklai <nadavh@marvell.com>
Cc: Neta Zur Hershkovits <neta@marvell.com>
Cc: Kostya Porotchkin <kostap@marvell.com>
Cc: Omri Itach <omrii@marvell.com>
Cc: Igal Liberman <igall@marvell.com>
Cc: Haim Boot <hayim@marvell.com>
Cc: Hanna Hawa <hannah@marvell.com>