drivers: of: of_slimbus: Register the slimbus slaves that are enabled

Currently all the child nodes of slimbus controller are registered even if
they are disabled. This leads to slimbus slave drivers getting probed even
if the slimbus slave devices are enabled.

Use for_each_available_child_of_node helper function to register only the
nodes that are enabled.

Change-Id: I930f5bf2ee4ddef7f74521bf3e8cc444b75f86e6
Signed-off-by: Karthikeyan Ramasubramanian <kramasub@codeaurora.org>