Add support for Macronix MX25U1635E, MX25U3235E/F and MX25U6435E/F.

Macronix MX25U3235 is a 4MB (32Mbits) 1.8v SPI flash supporting QPI.
Code for it was refined after merging it from chromium:
Change-Id: I62c7db070254ba3ec68090e783f57b25a6e8d15a
Reviewed-on: https://gerrit.chromium.org/gerrit/44395
Tested-by: Vincent Palatin <vpalatin@chromium.org>
Reviewed-by: Randall Spangler <rspangler@chromium.org>
Commit-Queue: Vincent Palatin <vpalatin@chromium.org>

Based on that support for MX25U1635E and MX25U6435E/F was added
by Stefan Tauner.
Also, add a feature flag for QPI-enabled chips: FEATURE_QPI.

Signed-off-by: Vincent Palatin <vpalatin@chromium.org>
Signed-off-by: Stefan Tauner <stefan.tauner@student.tuwien.ac.at>
Acked-by: Stefan Tauner <stefan.tauner@student.tuwien.ac.at>

git-svn-id: svn://coreboot.org/flashrom/trunk@1655 2b7e53f0-3cfb-0310-b3e9-8179ed1497e1