boston: Set CONFIG_SYS_LOAD_ADDR to 0x88000000

Generally we load Linux kernels on Boston boards in the form of FIT
images containing a compressed kernel binary. Linux is linked at
0x80100000 and so we need to decompress the kernel binary to that
address, however this is our default load address which means that
unless explicitly avoided we hit a decompression error as the
uncompressed kernel binary overwrites its compressed version from the
FIT image.

Avoid this by adjusting CONFIG_SYS_LOAD_ADDR to 0x88000000 (or
0xffffffff88000000 for MIPS64 builds) which avoids the address overlap
between compressed & uncompressed kernel binaries.

Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Cc: u-boot@lists.denx.de