Introudce wp_enable. To check WPSPL, wp_enable needs to be set
to 1 in board code.

Take i.MX6UL for example, for some boards, they do not use WP singal,
so they does not configure USDHC1_WP_SELECT_INPUT, and its default
value is 0(GPIO1_IO02). However GPIO1_IO02 is muxed for i2c usage and
SION bit set. So USDHC controller can always get wp signal and WPSPL
shows write protect and blocks driver continuing. This is not what
we want to see, so add wp_enable, and if set to 0, just omit the
WPSPL checking and this does not effect normal working of usdhc
controller.

If wp-gpios is provided in dts, wp_enable is set to 1, otherwise 0.

Signed-off-by: Peng Fan <van.freenix@gmail.com>
Cc: Pantelis Antoniou <panto@antoniou-consulting.com>
Cc: York Sun <york.sun@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Tested-by: Fabio Estevam <fabio.estevam@nxp.com>
Reviewed-by: York Sun <york.sun@nxp.com>