rk3288: Disable JTAG function from sdmmc0 IO

rk3288: Disable JTAG function from sdmmc0 IO

The GRF_SOC_CON0.grf_force_jtag bit is automatically set at boot and it
is preventing the SDMMC to work correctly. Disable the JTAG function on
the assumption that a working SD has higher priority over JTAG.

Signed-off-by: Carlo Caione <carlo@endlessm.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Acked-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>