ARM: shmobile: bockw: use wp-gpios instead of WP pin

Latest Renesas Chip has some SDHI channels and the WP pin
availability depends on its channel or HW implementation.
Thus, this patch decides new policy whch indicates
WP is disabled as default.
But, we can use wp-gpios property to enable it as
other method.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>