clk: fixed-factor: Add CLK_FIXED_FACTOR_FW_NAME for DT clock-names parent

With the new clk parenting code, clk_init_data was expanded to include
.parent_data, for clk drivers to specify parents using a combination of
device tree clock-names, pointers to struct clk_hw, device tree clocks,
and/or fallback global clock names.

Add a new macro, CLK_FIXED_FACTOR_FW_NAME, that takes a string to match
a clock-names entry in the device tree to specify the clock parent.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>