clk: imx6: Mask mmdc_ch1 handshake for periph2_sel and mmdc_ch1_axi_podf
MMDC CH1 is not used on i.MX6Q, so the handshake needed to change the
parent of periph2_sel or the divider of mmdc_ch1_axi_podf will never
succeed.
Disable the handshake mechanism to allow changing the frequency of
mmdc_ch1_axi, allowing to use it as a possible source for the LDB DI
clock.