ppc: Don't initialize write protected NOR flashes

If a NOR flash is write protected it can not be initialized/detected so
add the ability for boards to skip NOR initialization on bootup.  A
board can skip NOR initialization by implementing the
board_flash_wp_on() function.

Signed-off-by: John Schmoller <jschmoller@xes-inc.com>
Signed-off-by: Peter Tyser <ptyser@xes-inc.com>
CC: sr@denx.de
Acked-by: Stefan Roese <sr@denx.de>