ARM: dts: fix Cygnus nand device node

The third compatible string "brcm,brcmnand" in bcm-cygnus.dtsi nand node
is incorrect, redundant and should be removed. "brcm,brcmnand" is meant to
be used by STB based Broadcom SoCs. All iProc based SoCs should use
"brcm,nand-iproc".

Signed-off-by: Ray Jui <rjui@broadcom.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>