MIPS: Loongson: Make default kernel log buffer size as 128KB for Loongson3

When I update kernel with loongson3_defconfig based on the Loongson 3A3000
platform, then using dmesg command to show kernel ring buffer, the initial
kernel messages have disappeared due to the log buffer is too small, it is
better to change the kernel log buffer size from 16KB to 128KB which is
enough to save the boot messages.

Since the default LOG_BUF_SHIFT value is 17, the default kernel log buffer
size is 128KB, just delete the CONFIG_LOG_BUF_SHIFT line.

Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Paul Burton <paulburton@kernel.org>
Cc: paul.burton@mips.com
Cc: ralf@linux-mips.org
Cc: jhogan@kernel.org
Cc: chenhc@lemote.com
Cc: linux-mips@vger.kernel.org
Cc: linux-kernel@vger.kernel.org