clk: mediatek: return XTAL rate directly for gates with XTAL parent

clk: mediatek: return XTAL rate directly for gates with XTAL parent

There is currently a massive bug that makes any gate clk that have
CLK_XTAL as parent to return the wrong clock.

Following the code, with CLK_XTAL defined as TOPCKGEN parent, the
topckgen get_rate is called. The clk ID (0) is parsed and only in some
corner case (scenario where fixed clock are not defined) the correct
XTAL rate will be returned as get_factor or get_mux is called (that have
correct handling for CLK_XTAL). With fixed clock defined, the rate that
will be returned will always be the FIRST ELEMENT of the fixed clock
table instead of the hardcoded XTAL rate.

To handle this, add additional logic and if the flag is set to
PARENT_XTAL for the gate, return the XTAL rate directly.

We assume the clk_tree to have xtal_rate defined with clk gates that
have XTAL as parents.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>