mtd: brcmnand: Request and enable the clock if present

Attempt to enable a clock named "nand" as some SoCs have a clock for the
controller that needs to be enabled.

Signed-off-by: Simon Arlott <simon@fire.lp0.eu>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>