openrisc: Update litex defconfig to support glibc userland

I have been using a litex SoC for glibc verification.  Update the
default litex config to support required userspace API's needed for the
full glibc testsuite to pass.

This includes enabling the litex mmc driver and filesystems used
in a typical litex environment.

Signed-off-by: Stafford Horne <shorne@gmail.com>