imx8mq_evk_defconfig: Move file system options to Kconfig

Chris Spencer reports that when enabling ext4 read support without
also enabling write support the following error is seen:

fs/fs.c:198:12: error: 'ext4_write_file' undeclared here (not in a
function); did you mean 'ext4_read_file'?
   .write = ext4_write_file,
            ^~~~~~~~~~~~~~~
Fix this problem by moving these options to Kconfig.

Reported-by: Chris Spencer <spencercw@gmail.com>
Acked-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Fabio Estevam <festevam@gmail.com>