image: don't exceed gd->ram_top in bootm_size

image: don't exceed gd->ram_top in bootm_size

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:

Found /extlinux/extlinux.conf
Retrieving file: /extlinux/extlinux.conf
62 bytes read in 21 ms (2 KiB/s)
1:	linux
Retrieving file: /extlinux/Image
13740544 bytes read in 1266 ms (10.4 MiB/s)
Retrieving file: /extlinux/armada-8040-clearfog-gt-8k.dtb
33368 bytes read in 31 ms (1 MiB/s)
   Booting using the fdt blob at 0x4f00000
   Loading Device Tree to 00000000bfff4000, end 00000000bffff257 ... "Synchronous Abort" handler, esr 0x96000045
elr: 000000000006e1cc lr : 0000000000068fd8 (reloc)
elr: 000000007ffa91cc lr : 000000007ffa3fd8
x0 : ffffffffffffffff x1 : 00000000bfffc258
x2 : 0000000000000000 x3 : ffffffffffff7da7
x4 : 0000000004f08258 x5 : 00000000bfff4000
x6 : 00000000bfff4000 x7 : 000000000000000f
x8 : 000000007fb23bf8 x9 : 0000000000000008
x10: 00000000bffff257 x11: 00000000bffff257
x12: 0000000000000000 x13: fffffffffffff000
x14: 00000000bfff4000 x15: 0000000000000021
x16: 000000007ff7bc38 x17: 0000000000000000
x18: 000000007fb2add0 x19: 00000000bfff4000
x20: 0000000004f00000 x21: 000000000000b258
x22: 0000000058820000 x23: 0000000000000010
x24: 000000007ffe3c40 x25: 000000007fb23cb8
x26: 00000000c0000000 x27: 0000000000000000
x28: 000000007fc3fd50 x29: 000000007fb23bd0

Code: 54000061 aa0603e0 d65f03c0 38606882 (38206822)
Resetting CPU ...

Thanks to Patrice CHOTARD who directed me to the right way.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>