powerpc/ppc4xx: Make gdsys 405ep boards reset more generic

In order to add boards that have different hardware for fpga reset,
any 405ep gdsys board now provides these functions:

void gd405ep_init(void);
void gd405ep_set_fpga_reset(unsigned state);
void gd405ep_setup_hw(void);
int gd405ep_get_fpga_done(unsigned fpga);

Signed-off-by: Dirk Eibach <eibach@gdsys.de>
Cc: Stefan Roese <sr@denx.de>
Signed-off-by: Stefan Roese <sr@denx.de>