arm: k3: Correct an awk warning

The k3_gen_x509_cert.sh script produced this warning on gitlab and also
on my machine, e.g. with j7200_evm_r5:

awk: cmd. line:1: warning: regexp escape sequence `\ ' is not a known
   regexp operator

There is no need to escape spaces, so drop the backslashes. Also split
the line so it is a more reasonable length.

This script should really be deleted and binman used instead.

Signed-off-by: Simon Glass <sjg@chromium.org>