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

As a part of __show_regs we print the 512 bytes of data around
key registers like SP, PC and LR. These data is not always useful
and can cause issue while we have more cores in the system and need
to print them per core during the stack traces triggered through
WARN(..) like APIs. It can also cause latency issues due to heavy
printing on the serial console.

Reduce the logging to 128 bytes (nbytes * 2) around these registers
from 512 bytes.

Change-Id: Ie19fececd9d2bc6cd3328a6c63c956bcc9eed9a6
Signed-off-by: Trilok Soni <tsoni@codeaurora.org>