powerpc: 74xx_7xx: remove 74xx_7xx cpu support

All the 74xx_7xx boards are still non-generic boards:
P3G4, ZUMA, ppmc7xx, ELPPC, mpc7448hpc2

Acked-by: Marek Vasut <marex@denx.de>
Acked-by: Stefan Roese <sr@denx.de>
Acked-by: York Sun <yorksun@freescale.com>
Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Wolfgang Denk <wd@denx.de>
Cc: Nye Liu <nyet@zumanetworks.com>
Cc: Roy Zang <tie-fei.zang@freescale.com>