mtd: nand: add NAND driver "library" for Broadcom STB NAND controller

This core originated in Set-Top Box chips (BCM7xxx) but is used in a
variety of other Broadcom chips, including some BCM63xxx, BCM33xx, and
iProc/Cygnus. It's been used only on ARM and MIPS SoCs, so restrict it
to those architectures.

There are multiple revisions of this core throughout the years, and
almost every version broke register compatibility in some small way, but
with some effort, this driver is able to support v4.0, v5.0, v6.x, v7.0,
and v7.1. It's been tested on v5.0, v6.0, v6.1, v7.0, and v7.1 recently,
so there hopefully are no more lurking inconsistencies.

This patch adds just some library support, on which platform drivers can
be built.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Tested-by: Florian Fainelli <f.fainelli@gmail.com>