disk/part.c: Expose list of available block drivers

We have a pretty nice and generic interface to ask for a specific block
device. However, that one is still based around the magic notion that
we know the driver name.

In order to be able to write fully generic disk access code, expose the
currently internal list to other source files so that they can scan through
all available block drivers.

Signed-off-by: Alexander Graf <agraf@suse.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Simon Glass <sjg@chromium.org>