dm: clk: Remove simple version of clk_get_by_index/name()

Simple version of clk_get_by_index() added by:
"dm: clk: Add a simple version of clk_get_by_index()"
(sha1: a4b10c08)
is only working for #clock-cells=<1> but not for
any other values. Fixed clocks is using #clock-cells=<0>
which requires full implementation.

Remove simplified versions of clk_get_by_index() and use full version.
Also remove empty clk_get_by_name() which is failing when it is called
which is useless.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Acked-by: Stephen Warren <swarren@nvidia.com>