vboot: Increase max parsed fw components to 6

With addition of bl31 and trusty, we need to increase the number of
parsed fw components in vboot to 6.

CQ-DEPEND=CL:273865
BUG=chrome-os-partner:40713
BRANCH=None
TEST=Compiles successfully and vboot finds trusty and bl31.

Change-Id: I9b3e59a6d29395761fda5d15cdf854c446d2bf04
Signed-off-by: Furquan Shaikh <furquan@google.com>
Reviewed-on: https://chromium-review.googlesource.com/273866
Tested-by: Furquan Shaikh <furquan@chromium.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>
Commit-Queue: Furquan Shaikh <furquan@chromium.org>
Trybot-Ready: Furquan Shaikh <furquan@chromium.org>