clk: uclass: clk_get_by_name() must not be available if CONFIG_OF_PLATDATA is enabled

clk_get_by_name() requires clk_get_by_id() that is not available if
CONFIG_OF_PLATDATA is defined, so move clk_get_by_name() into #else
condition of #if CONFIG_IS_ENABLED(OF_PLATDATA).

Signed-off-by: Giulio Benetti <giulio.benetti@benettiengineering.com>