defconfig: msm: Enable RCU_FAST_NO_HZ for sdm855

Enable RCU fast for better performance and to have
same config flags across sdm855_defconfig and
sdm855-perf_defconfig files.

Change-Id: I442101422c6fdc541c170f7c8bed476f39525ec9
Signed-off-by: Prasad Sodagudi <psodagud@codeaurora.org>