ppc: Don't initialize write protected NOR flashes

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 <[email protected]>
Signed-off-by: Peter Tyser <[email protected]>
CC: [email protected]
Acked-by: Stefan Roese <[email protected]>