When board_get_usable_ram_top() limits gd->ram_top, env_get_bootm_size()
must not exceed that limit. Otherwise, boot_relocate_fdt() might put fdt
out of the allowed RAM range.
The similar commit 8ce1f10cf2b1 ("ARM: bootm: take into account
gd->ram_top") exposed this bug.
This fixes boot on Armada 8040 based Clearfog GT-8K where ram_top is set
to 0x80000000 (2GB), but bi_dram[0].size might be up to 0xc0000000
(3GB). Note the relocated fdt address (0xbfff4000) in the console output
listed below: