tools: gen_eth_addr: add getpid() to time(0) to avoid duplicated seed

As 'time(0) | getpid()' will have a lot of duplicated value. It is not a
expected behavior. We expect different value for the seed when when run
it in many times.

So this patch will left shift the getpid() and add to time(0). That
avoid duplicated value.

Test command is like:
  % RUN=0; while [ $RUN -lt 10000 ]; do
  tools/gen_eth_addr; RUN=$(($RUN+1)); done | sort | uniq | wc -l
  10000

This patch is incorporated with suggestions made by Wolfgang Denk and Andreas
Bießmann. Thanks them a lot.

Signed-off-by: Josh Wu <josh.wu@atmel.com>
Acked-by: Andreas Bießmann <andreas.devel@googlemail.com>
Acked-by: Wolfgang Denk <wd@denx.de>
Tested-by: Wolfgang Denk <wd@denx.de>