spi: add config option to enable the WP pin function on st micron flashes

enable the W#/Vpp signal to disable writing to the status
register on ST MICRON flashes like the N25Q128 thorugh
the new config option CONFIG_SYS_SPI_ST_ENABLE_WP_PIN

Signed-off-by: Heiko Schocher <hs@denx.de>
Cc: Jagannadha Sutradharudu Teki <jagannadh.teki@gmail.com>