ARM: dts: m28evk: Fix MMC WP pin being inverted

The MMC WP is active high on the m28evk, fix this.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>