habanalabs: move driver to accel subsystem

Now that we have a subsystem for compute accelerators, move the
habanalabs driver to it.

This patch only moves the files and fixes the Makefiles. Future
patches will change the existing code to register to the accel
subsystem and expose the accel device char files instead of the
habanalabs device char files.

Update the MAINTAINERS file to reflect this change.

Signed-off-by: Oded Gabbay <ogabbay@kernel.org>