clk: mediatek: add CLK_BYPASS_XTAL flag to allow bypassing searching clock parent of xtal clock

The mtk clock framework in u-boot uses array index for searching clock
parent (kernel uses strings for search), so we need to specify a special
clock with ID=0 for CLK_XTAL in u-boot.

In the mt7622/mt7629 clock tree, the clocks with ID=0 never call
mtk_topckgen_get_mux_rate, adn return xtal clock directly. This what we
expected.

However for newer chips, they may have some clocks with ID=0 not
representing the xtal clock and still needs mtk_topckgen_get_mux_rate be
called. Current logic will make entire clock driver not working.

This patch adds a flag to indicate that whether a clock driver needs clocks
with ID=0 to call mtk_topckgen_get_mux_rate.

Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Daniel Golle <daniel@makrotopia.org>
Signed-off-by: Weijie Gao <weijie.gao@mediatek.com>