mips_malta: correct reading MIPS revision at 0x1fc00010

mips_malta: correct reading MIPS revision at 0x1fc00010

Rather than modifying the BIOS code at its original location, copy it
for the 0x1fc00000 region & modify the copy. This means the original
ROM code is correctly readable at 0x1e000010 whilst the MIPS revision
is readable at 0x1fc00010.

Additionally the code previously operated on target memory which would
later be overwritten by the BIOS image upon CPU reset if the -bios
argument was used to specify the BIOS image. This led to the written
MIPS revision being lost. Copying using rom_copy when -bios is used
fixes this issue.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Signed-off-by: Leon Alrae <leon.alrae@imgtec.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>