clk: refactor clk_get_by_index() into clk_get_by_indexed_prop()

The logic in clk_get_by_index() may be useful for other properties
than 'clocks': e.g. 'assigned-clocks' and 'assigned-clock-parents'
follows the same model.

This commit refactors clk_get_by_index() by introducing an internal
function clk_get_by_indexed_prop() that allows to specify the name
of the property to process.  The original clk_get_by_index() call
is simply directed through this helper function with the property
name fixed to "clocks".

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Tested-by: David Wu <david.wu@rock-chips.com>

Series-changes: 2
- Fixed David's email address.