MLK-14706 clk: imx7d: align with upstream: update arm a7 root clock init

MLK-14706 clk: imx7d: align with upstream: update arm a7 root clock init

In upstream, IMX7D_ARM_A7_ROOT_SRC uses imx_clk_mux2 for initialization,
while our code uses imx_clk_mux_flags_bus with additional flags
CLK_SET_RATE_NO_REPARENT | CLK_OPS_PARENT_ENABLE. In the end, both calls
set the flags of IMX7D_ARM_A7_ROOT_SRC clock to
CLK_SET_RATE_NO_REPARENT | CLK_OPS_PARENT_ENABLE.

To align our code to upstream, we can safely use imx_clk_mux2.

Signed-off-by: Irina Tirdea <irina.tirdea@nxp.com>