clk: sunxi-ng: mux: honor CLK_SET_RATE_NO_REPARENT flag

This patch adds support for the CLK_SET_RATE_NO_REPARENT flag to the
mux determine_rate helper, based on examples from clk-composite and
clk-mux.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>