drm: atmel-hlcdc: Add pinctrl PM select sleep,default state in CRTC suspend/resume

Some LCD panels have back-powering issue when un-powered, allows users
to use an alternate pinctrl "sleep" in order to clamp outputs to a
wanted state at suspend.

Signed-off-by: Sylvain Rochet <sylvain.rochet@finsecur.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>