clk: imx6ull: use OSC clock during AXI rate change

On i.MX6 ULL using PLL3 seems to cause a freeze when setting
the parent to IMX6UL_CLK_PLL3_USB_OTG. This only seems to appear
since commit 6f9575e5 ("clk: imx: Add CLK_IS_CRITICAL flag
for busy divider and busy mux"), probably because the clock is
now forced to be on.

Fixes: 6f9575e5("clk: imx: Add CLK_IS_CRITICAL flag for busy divider and busy mux")
Signed-off-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>