mtd: rawnand: stm32_fmc2: add NAND Write Protect support

This patch adds the support of the WP# signal. WP will be disabled
before the first access to the NAND flash.

Signed-off-by: Christophe Kerello <christophe.kerello@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>