dt-bindings: mtd: brcmnand: Add WP pin connection property

Add brcm,wp-not-connected property to have an option for disabling this
feature on broadband board design that does not connect WP pin.

Signed-off-by: William Zhang <william.zhang@broadcom.com>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20240223034758.13753-6-william.zhang@broadcom.com