LoongArch: defconfig: Set CONFIG_TXGBE=m by default

LoongArch inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I5OHOB



--------------------------------

Ensure the netswift 10G NIC driver ko can be distributed in ISO on
LoongArch.

Signed-off-by: Hongchen Zhang <zhanghongchen@loongson.cn>