arm64: process: Reduce the no. of bytes of data printed around registers

Since ARMv8 has more registers and printing 256bytes of data
around these registers will fill up the dmesg with the SoCs having
eight processors and it will be hard to analyze the original reason
of the crash since those useful backtrace information is overwritten.

Reduce the no. of bytes to half of what we are printing right now
to improve collection of useful information during the panic.

Change-Id: I5761e393b23c436907d255339c53eeaf25c081c1
Signed-off-by: Trilok Soni <tsoni@codeaurora.org>