mpc8xx: move common linker scripts into the CPU directory

mpc8xx: move common linker scripts into the CPU directory

Each CPU of PowerPC has its default linker script under the CPU
directory, except mpc8xx.

Every mpc8xx board has its own linker script under the board
directory, resulting in lots of duplication of linker scripts.

I notice eight mpc8xx boards have the same linker script.
We can decrease the number of linker scripts by putting a single
default linker script, arch/powerpc/cpu/mpc8xx/u-boot.lds.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Wolfgang Denk <wd@denx.de>
Acked-by: Stefan Roese <sr@denx.de>