imx: sksimx6_defconfig: remove SPL misc option to fix build error

Remove CONFIG_SPL_DRIVERS_MISC_SUPPORT since it is already defined
in include/configs/mx6_common.h for SPL builds. When this option
is in defconfig, we get build warnings (or errors when building
with -Werror).

Signed-off-by: Anatolij Gustschin <agust@denx.de>