rockchip: mkimage: remove placeholder functions from rkimage

The imagetool framework checks whether function pointer for the verify,
print and extract actions are available and will will handle their
absence appropriately.

This change removes the unnecessary functions and uses the driver
structure to convey available functionality to imagetool.  This is in
fact better than having verify just return 0 (which previously broke
dumpimage, as dumpimage assumed that we had handled the image and did
not continue to probe further).

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Acked-by: Simon Glass <sjg@chromium.org>