Documentation: devicetree: brcmstb_nand: add BCM63138 and Cygnus/iProc

Add notes to describe how some SoCs integrate this NAND core a little
differently, and so we define extra compatibility strings and register
resources/names.

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