stm32mon: set flash_size to 128KB for chip ID 0x420

This updates the flash_size to match 128KB parts. Unfortunately
there does not seem to be an easy way to differentiate between the
64KB and 128KB parts at runtime.

BUG=none
TEST=tested on Snow

Signed-off-by: David Hendricks <dhendrix@chromium.org>

Change-Id: Ie1ca6f6d04753e91d937f67dec193fcf5566251e
Reviewed-on: https://gerrit.chromium.org/gerrit/26188
Reviewed-by: Vincent Palatin <vpalatin@chromium.org>
Commit-Ready: David Hendricks <dhendrix@chromium.org>
Tested-by: David Hendricks <dhendrix@chromium.org>