powerpc/mpc8536ds: Invert SDHC_WP pin polarity

MPC8536 Rev 1.0 silicon have NMG_eSDHC118 erratum, so that the SDHC write
protected pin polarity does not follow the SD card standard in MPC8536
Rev 1.0 silicon.

The MPC8536DS board invert the SDHC_WP pin as a workaround.  However, this
silicon erratum has been fixed in Rev 1.1, So need invert the SDHC_WP
polarity again when use the MPC8536 Rev1.1 and greater on MPC8536DS board.

Signed-off-by: Xie Xiaobo <r63061@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>