ARM bootm regression

[Alexander Shiyan]

Hello.

Found a regression in arm/lib32/bootm.c after a patch
"ARM: bootm: make sure we place the kernel in free memory".

The initial memory size for the board is set to 8 MB (before reading the real value from DT),
so the stack, malloc_space, etc. are placed in the initial 8M space.
Then get_kernel_addresses() gets the real SDRAM area (for my board it is 64M),
but decreases this size by the first children size (malloc_space),
so mem_end is incorrect in this case.

barebox 2020.05.0-00393-gf5e96a296-dirty #80 Thu Oct 15 13:08:54 MSK 2020
Board: Mega-Milas Informer EP7312
cfi_flash 90000000.nor at 0,0.of: found cfi flash at 0x90000000, size 32 MiB
i2c-gpio i2c0.of: using pins 16 (SDA) and 35 (SCL)
Hit any to stop autoboot:    1
Booting entry 'nor'
Loading ARM Linux zImage '/dev/nor0.kernel'
__request_region: 0xbfdc6000:0xc00fca17 outside parent resource 0xc0000000:0xc3ffffff
ERROR: bootm/zImage: failed to request memory at 0xbfdc6000 to 0xc00fca18 (3369496).
ERROR: Booting entry 'nor' failed
Nothing bootable found

---