mmc: tmio: use generic GPIO CD and WP handlers

The tmio-mmc driver is already using the generic GPIO CD handler in IRQ
mode. This patch adds support for CD polling mode and also checks for
availability of a WP GPIO.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Chris Ball <cjb@laptop.org>