clk: imx6sl: ensure MMDC CH0 handshake is bypassed

Same as other i.MX6 SoCs, ensure unused MMDC channel's
handshake is bypassed, this is to make sure no request
signal will be generated when periphe_clk_sel is changed
or SRC warm reset is triggered.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>