mips: wdr4300: Move the CONFIG_USE_PRIVATE_LIBGCC to Kconfig

This fixes the last remaining libgcc warning, where the symbol was
defined twice.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Simon Glass <sjg@chromium.org>
Cc: Tom Rini <trini@konsulko.com>
Reviewed-by: Tom Rini <trini@konsulko.com>