powerpc: ppc4xx: convert AMCC boards to generic board

Add CONFIG_SYS_GENERIC_BOARD to amcc-common.h and CONFIG_DISPLAY_BOARDINFO
to Kconfig files. canyonlands.h includes amcc-common.h, so remove
CONFIG_SYS_GENERIC_BOARD definition there.

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Cc: Stefan Roese <sr@denx.de>
Cc: Feng Kan <fkan@amcc.com>
Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
Cc: Tom Rini <trini@konsulko.com>