mtd: brcmnand: add extra SoC support to library

There are a few small hooks required for chips like BCM63138 and the
iProc family. Let's introduce those now.

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