arm64: defconfig: Do not lower CONFIG_LOG_BUF_SHIFT

The default value of 17 for CONFIG_LOG_BUF_SHIFT is much more suitable
than 14. The latter easily leads to lost kernel messages on systems with
only one CPU core.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>