clk: imx6sll: fix missing of_node_put()

of_find_compatible_node() is returning a device node with refcount
incremented and must be explicitly decremented after the last use
which is right after the us in of_iomap() here.

Signed-off-by: Nicholas Mc Guire <hofrat@osadl.org>
Fixes: 4a5f720b ("clk: imx: add clock driver for imx6sll")
Signed-off-by: Stephen Boyd <sboyd@kernel.org>