of: dynamic: Fix incorrect parameter name and provide missing descriptions

Fixes the following W=1 kernel build warning(s):

 drivers/of/dynamic.c:234: warning: Function parameter or member 'np' not described in 'of_attach_node'
 drivers/of/dynamic.c:286: warning: Function parameter or member 'np' not described in 'of_detach_node'
 drivers/of/dynamic.c:326: warning: Function parameter or member 'kobj' not described in 'of_node_release'
 drivers/of/dynamic.c:326: warning: Excess function parameter 'kref' description in 'of_node_release'

Cc: Rob Herring <robh+dt@kernel.org>
Cc: Frank Rowand <frowand.list@gmail.com>
Cc: devicetree@vger.kernel.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210318104036.3175910-3-lee.jones@linaro.org