clk: Document CLK_MUX_READ_ONLY mux flag

This flag isn't documented. Document it.

Signed-off-by: Stephen Boyd <sboyd@kernel.org>

include/linux/clk-provider.h

@@ -499,6 +499,8 @@ void clk_hw_unregister_divider(struct clk_hw *hw);
  *	register, and mask of mux bits are in higher 16-bit of this register.
  *	While setting the mux bits, higher 16-bit should also be updated to
  *	indicate changing mux bits.
+ * CLK_MUX_READ_ONLY - The mux registers can't be written, only read in the
+ * 	.get_parent clk_op.
  * CLK_MUX_ROUND_CLOSEST - Use the parent rate that is closest to the desired
  *	frequency.
  */