boards: remove usage of DT_LABEL

A number of boards utilize device_get_binding(DT_LABEL(...)) to
get a gpio for some purpose.  Switch over to using DEVICE_DT_GET
and device_is_ready() instead.  This is part of the ongoing
cleanup towards phasing out usage of the "label" property in DTS.

Signed-off-by: Kumar Gala <galak@kernel.org>