powerpc: mpc8260: consolidate CONFIG_MPC8260 and CONFIG_8260

Before this commit, CONFIG_MPC8260 and CONFIG_8260
were used mixed-up.

All boards with mpc8260 cpu defined both of them:
  - CONFIG_MPC8260 was defined in board config headers
      and include/common.h
  - CONFIG_8260 was defined arch/powerpc/cpu/mpc8260/config.mk

We do not need to have both of them.
This commit keeps only CONFIG_MPC8260.

This commit does:
 - Delete CONFIG_8260 and CONFIG_MPC8260 definition
   in config headers and include/common.h
 - Rename CONFIG_8260 to CONFIG_MPC8260
    in arch/powerpc/cpu/mpc8260/config.mk.
 - Rename #ifdef CONFIG_8260 to #ifdef CONFIG_MPC8260

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