ar71xx: add preliminary kernel support for several RB SPI NOR devices

ar71xx: add preliminary kernel support for several RB SPI NOR devices

The cAP 2nD, mAP 2nD and wAP 2nD apparently all share the same QCA953x/SPI-NOR
platform.

This patch adds preliminary kernel support based on init data used in
Mikrotik's linux-3.3.5 kernel patch. Since this is totally experimental and
untested, the necessary glue in base-files has not been added, the code is
provided as a starting point in case somebody with that hardware wants to help
getting it supported.

The cAP lite (cAPL 2nD) should also be supported once it is determined whether
or not it uses the same board identifier as the mAP lite.

Code has been successfully built and has no side effect (no impact on already
supported devices).

Signed-off-by: Thibaut VARENE <hacks@slashdirt.org>