Add board_flash_wp_on() to check a pca9557 gpio pin to see
if non-volatile memory write protection is enabled.

Previously, write protected NOR flashes would fail initialization which
resulted in a bootup error such as:

 ...
 DTT:   53 C local / 64 C remote (adt7461@4c)
 DTT:   54 C local (ds1621@48)
 FLASH: Executed from FLASH1
 POST memory PASSED
 FLASH: ## Unknown FLASH on Bank 1 - Size = 0x00000000 = 0 MB
 ## Unknown FLASH on Bank 2 - Size = 0x00000000 = 0 MB
 *** failed ***
 ### ERROR ### Please RESET the board ###

With this patch, NOR flash initialization is skipped:

 ...
 DTT:   53 C local / 64 C remote (adt7461@4c)
 DTT:   54 C local (ds1621@48)
 FLASH: Executed from FLASH1
 POST memory PASSED
 FLASH: Uninitialized - Write Protect On
 L2:    1024 KB enabled
 NAND:  1024 MiB
 ...

Note that flash related commands such as flinfo and saveenv will error
out when flash write protection is enabled.

Signed-off-by: John Schmoller <jschmoller@xes-inc.com>
Signed-off-by: Peter Tyser <ptyser@xes-inc.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>