PowerPC: drop some 74xx_7xx boards and related code

The file  board/Marvell/include/mv_gen_reg.h  is incompatible with
the GPL (see for example the "MARVELL RESERVES THE RIGHT AT ITS SOLE
DISCRETION TO REQUEST THAT THIS CODE BE IMMEDIATELY RETURNED TO
MARVELL" clause).  As this cannot be fixed, we remove the file and all
code that depends on it.  Fortunately this only affects some very old
boards that have long reached EOL:
	CPCI750
	DB64360
	DB64460
	p3m750
	p3m7448

Signed-off-by: Wolfgang Denk <wd@denx.de>
Cc: Reinhard Arlt <reinhard.arlt@esd-electronics.com>
Cc: Stefan Roese <sr@denx.de>
Cc: Roger Meier <r.meier@siemens.com>