ti816x_evm: Disable CONFIG_USE_PRIVATE_LIBGCC

On this platform, we can trace a general failure to boot to enabling /
disabling this option.  When this is enabled, we go off into the
weeds during SPL and are unable to talk with the SD card and
mmc_initialize() fails.

Signed-off-by: Tom Rini <trini@konsulko.com>