arm64: mvebu: handle non-zero base address for RAM

board_get_usable_ram_top() conflated the RAM size with the top address
of RAM. On systems where RAM starts at address 0 these numbers are the
same so it went unnoticed. Update board_get_usable_ram_top() to take
CONFIG_SYS_SDRAM_BASE into account when determining the top address.

Signed-off-by: Chris Packham <judge.packham@gmail.com>
Reviewed-by: Stefan Roese <sr@denx.de>