clk: imx6sx: Register SAI clocks as shared clocks

SAIx and SAIx_IPG share the same bit fields in the CCM registers, so
we should better register them via imx_clk_gate2_shared().

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>