m68k/amiga: Chip RAM - Always allocate from the start of memory

As of commit 5df1abdb ('m68k/amiga: Fix
"debug=mem"'), "debug=mem" no longer uses amiga_chip_alloc_res(), so we
can remove the hack to prefer memory at the safe end.

This allows to simplify the code and make amiga_chip_alloc() just call
amiga_chip_alloc_res() internally.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>